The SEC basically allows the schools to govern themselves, in essence each school has to have "a policy." The move to 4 strikes (from 3) was more or less to just level the playing field and be more consistent with the rest of the conference, it wasn't really a cheap shot to try and get guys back on the field.
From what I read, if you admit that you're going to fail a test that doesn't count against your "strikes"; and if you have a previous positive test, if your most RECENT positive test is less % value then that also does not count against you.
Basically, an athlete who smokes 100 joints per day could be tested once per month, and as long as "x" athlete smokes less joints per month it's okay because he's making progress.
I may be off on the exact wording behind this, but that's basically the theory behind it.