When the bulk are the dregs of society, your "picky" tends not to be too great. Believe me, in that emergency buildup we did early in the war we let in some real nasty folks--lots of troublemakers. Drugs, sexual assault, gangs, you name it. Conversely, we had some real weird problems with tattoo placement rules eliminating solid people from joining.
Anyway, we don't exactly have a "Starship Troopers" (book, not movie) training philosophy here. Basic training is hard, but it's not that hard, and it's difficult to quit. When someone joins up and they have even a modicum of desire to be there, they'll be in and that's it. If it turns out they're dirtbags, then it's very difficult to get rid of them. It used to be damn near impossible, but the drawdown brought about many self-selecters.
I thought the tattoo issues arose after they no longer needed so many people that they were overlooking felonies; I didn’t think these were simultaneous events. I also remember the “blue to green” program, where it seemed they coerced many long-serving Air Force and Navy personnel into switching to Army and Marine units to get more “boots on the ground”.