Yes. Because operating outside of the law is so much fun, everyone wants to do it!
Edited for DRAMA!
No, only about 50% of them.
Actually, you should re-read the article. It's more about the increasing efficiency in identifying illegal immigrants in US jails and prisons so they can be deported, than an actual study on the proportion of criminal illegals.
There isn't a direct relation between the fact that 50% of the deported illegal immigrants last year had criminal records and the actual proportion of illegal immigrant criminals. After all, it makes sense that criminals have more chances of being deported than non-criminals.
EDIT: damn I'm dense today.
I understand the intention of the article, I just thought it had a little tie-in to your statement. However, I dont understand your second paragraph. The way I read that is, out of all the people deported last year (not just the ones already in jail) roughly 50% had existing criminal records. IMO, I feel that does show some cause and effect. People already predisposed to crime have no qualms about breaking more laws to further their own gain/status (such as illegally immigrating).