The Wellness Policy applies to 'regulars' and that was thought up to allow local jobbers not to be tested. Guys who are cans and come in to be squashed for one night.
Brock is arguably the biggest star and is constantly constantly in advertising. He only shows up at limited dates... But that's the same for everybody. Nobody plays every show. Most appear 8-10 times more but there's no fixed limit.
Vince used the ambiguity of 'regular' to get out of his fucked situation.
The regular guy who was being pushed to be the biggest star (and fans hated) popped under WWE testing. This was huge huge news. 'Luckily' his suspension allowed him to appear at his billed PPV.
Brock is headlining the second biggest event of the year with a returning superstar (injury) - Summerslam. If Brocl suspended for the standard 30 days then that wouldn't matter, he'd still appear but a) Brock would be pissed and maybe noshow b) WWE pretends that what happens outside of the WWE doesn't matter. Having the chosen one plus Brock looks very very bad on tv, especially after the chosen one got major shit on tv and from fans.
This does make Wellness a bit of a mockery but until the UFC started with USADA the WWE had the most comprehensive and hardcore (random, blood and piss) testing regime of them both and that's for almost a decade. Fail three tests and you're out.