Sure, most SEC complaints are for lack of transparency, because they don't have laws against a lot of the stuff they want to stop. But they don't go around prosecuting for lack of transparency of good things. If they fined someone for it, it's probably because they don't approve of the thing they did, which is a good sign they're going to fine others that do it.

And it's not like a lot of internalizers are going around with prospectuses saying "we're gonna purposefully move markets to give your customers worse prices", so the SEC isn't going to have much trouble fining them if they decide to (assuming this is actually happening all the time).