Why is it that when something bad happens, Christians (in particular) are content with the idea that God's ways are inscrutable, but somehow those ways are perfectly knowable when the events are advantageous?

Here's a particularly egregious example.  Baltimore Ravens linebacker Ray Lewis was indicted in 2000 on two counts of murder and aggravated assault.  He struck a plea deal with the prosecutor in which he testified against two friends who were also charged, and Lewis was convicted only of obstructing justice, for which he received probation.  Neither of the friends was ultimately found guilty of the murder charges, and to this day, Lewis has never explained what happened, what his role was in the murders, or why he lied to the police.  He has six children by four different women.  And he is also a vociferous and often tearful Christian.  In a pre-SuperBowl softball interview with his former teammate Shannon Sharpe, Lewis was gently asked about the events of 2000.  His answer went like this:
I am a great football player.  (Granted)
All my football skills testify to the glory of God.
God wouldn't waste his glory on those who are unworthy.
Therefore, I deserve all that I have accomplished.

Ingenious, if not ingenuous.

When I wrote about Flannery O'Connor's "Good Country People," my point was that the Catholic O'Connor reversed the traditional argument against the existence of God--the universality of evil in the world--to use it for her religious beliefs.  O'Connor doesn't deny the prevalence of evil.  Rather, she argues that nihilism/atheism leaves one defenseless against it.  Therefore, since good also exists, there must be a god to account for goodness and to provide defense against evil.

In his famous sonnet "Design," Robert Frost uses a similar tactic for the opposite end: he takes one of the most traditional arguments for the existence of God, the argument from design, and turns it on its head: 

I found a dimpled spider, fat and white,
On a white heal-all, holding up a moth
Like a white piece of rigid satin cloth --
Assorted characters of death and blight
Mixed ready to begin the morning right,
Like the ingredients of a witches' broth --
A snow-drop spider, a flower like a froth,
And dead wings carried like a paper kite.

What had that flower to do with being white,
The wayside blue and innocent heal-all?
What brought the kindred spider to that height,
Then steered the white moth thither in the night?
What but design of darkness to appall?--
If design govern in a thing so small.
      
Immediately, the choice of a sonnet, one of the most traditional and carefully designed of all poetic forms, reinforces the sense of design.  The octave itself has a symmetrical structure--three lines of description, two lines of comment, and three lines of simile.  The first three lines describe a morning breakfast scene enacted by three white characters.  (Frost always prefers synecdoche to symbolism, metonymy to metaphor; these three are not symbols: they're a representative part of the larger natural world.)  White, used three times in three lines, traditionally indicates purity,  The use of 'dimpled' to describe the spider may throw the reader for a moment, and even if we've never seen the flower known as the heal-all, its name suggests its beneficial powers.  But Frost doesn't allow for ambiguity for long, as in lines four and five he makes explicit the nature of his characters with the strange comment that predation and death is the way to "begin the morning right"--not what we would normally assume.  Moreover, both of the last two words have homonyms: is it to begin the morning right or the mourning right?  is it 'right' or 'rite'?  Certainly there appears to be a ritual aspect to the rigid wings, as if we're watching a black mass with the priest holding up the sacrificial 'host,' though sacrificial to no public good.  And then there are the three lines of similes, again a mixture of the deceptively pleasant (snow-drop spider, froth, paper kite) with the fatal reality of one character's survival at the cost of another's life.

The scene described, the sestet begins with the philosophical/religious question about the role of design.  Frost doesn't ask "who" has scripted this scene, but "what?"  What brought the innocent heal-all, more commonly blue than white, to participate as the stage, the altar for the spider's predation?  The spider, once 'dimpled' and then 'snow-drop,' is now 'kindred' to the heal-all; they are both participating in the deadly ritual.  And finally, what brought the moth to its death.  Frost is shrewd in his choice of a moth; no matter how much we love wildlife, moth protection is probably not high on our list of charities.  As we spend twelve lines watching the observing narrator describe the death scene, we probably do so from a rather detached perspective.  Frost doesn't deign to cheapen the poem with a more emotionally involving death. 

Like most Italian sonnets written after Shakespeare introduced an alternative form, this varies from the original Petrarchan structure and borrows Shakespeare's use of a final couplet.  Line thirteen makes clear the poet's attitude toward an argument from design: after twelve carefully sculpted lines whose content as well as form hints strongly at design, Frost suggests that it's a "design of darkness" and that we ought to find any such design appalling.  And then, in his brilliant last line, Frost casually tosses off an alternative reading of the scene: perhaps there is no design at all.  This last possibility, so frightening to many of the religious, is here just an off-hand remark.  Design of darkness or no design at all--take your choice.