@ Intractable Potsherd
If they had the faintest reason to believe there was an actual explosive device inside a given piece of luggage then it simply wouldn't be opened at all.
When they open them they're looking for contraband, and they cut them open because it's fast and there appears to be no consequences whatsoever for them destroying peoples' property.
I saw a recent TV documentary about US customs. The staff shown in that one seemed to take pride in wanton destruction - and had a rather worrying habit of waving sharp knives around.
If anybody else did that it would be (possibly malicious) criminal damage.
The result? Travel insurance via and to the US is considerably higher than going anywhere else in the world, and people avoid the US whenever possible.
BTW, American Airlines in is Chapter 11 (administration). This may be related...
Anon because I do occasionally have to travel there.