It's not surprising that the WWE would protect their investment in Reigns. This triple threat match / angle where the Shield faces off is a big deal and something that may not happen again for a long time. If they really felt the need to punish Reigns, they still could once the match was done.
In all likelihood, beyond the stipulated 30 days, the WWE will temper any further fallout based on circumstances. If he has no history of trouble with the company and he wasn't caught using illegal narcotics, don't be surprised if his punishment isn't severe. And if it turns out it was a prescribed medication, but Reigns didn't do the work to make sure the office got a notice properly from his physician, I would expect no real punishment beyond loss of pay for the duration of the 30 day suspension.
Even had the PPV fallen on day 20 of the suspension, I suspect they'd still have him work the match. Several years ago, those Wellness suspensions had affected PPVs to the point they changed up enforcement. I believe they had Orton or someone of that caliber work a PPV date, but wasn't paid his usual bonuses.