Well, I do agree that it should be fairly easy to exclude those people who do not want to work from those who are just temporarily down on their luck. I think drug testing would be OK provided:
1) Benefits were raised somewhat.
2) The applicant didn't have to pay for the drug test the first time, or ever so long as the tests come back clean. Someone who fails a test will pay for their next test, and future tests until they're clean.
3) There was an exception for light marijuana use.
That said, I'm not sure I understand the relevance. The bombers are not accused of dealing or using drugs. They're accused of blowing people up and shooting them. Currently, there does not seem to be any indication that drug use led to their crimes. That is, there's no indication that they were sitting around a shabby basement, freebasing some crystal meth, when suddenly one says to the other "Dude! Like, you know what would be so cosmic and awesome? Check it, like, if we blow some **** up and killed some people, that would be, like, sick, man!"
They appear to have had other concerns.