Not legitimate in which way? And how hard would it be to make it so the karma system was fixed to allow for this properly?
As far as I know, there really is no way to detect prop pushing and non-karmable prop killing properly. It's not legitimate for obvious reasons lol, you won't gain or lose any karma or score for doing so, and your karma doesn't account into the kill either. So, if you have really low karma, and you decided "WELP, I CAN'T KILL ANYTHING, GUESS I'LL THROW SOME PROPS AROUND" then you can just easily kill someone and your low karma won't reduce the damage to the other player in any way.
one more question: can anyone crush explosive barrel to people and blow people up?
No, that's the same as non-karmable prop killing. It just blows the barrel up and it doesn't detect who did it, so no one loses any karma or blah blah blah same shit as above.