-Game encourages maxxing in S2 Acts too with same bonus
That is irrelevant, as those are completely normal Acts and you aren't supposed to go out of your way to collect every Ring in the level, even if it's possible to do that.
The perfect bonus earned by collecting all of the available Rings in the S3&K Special Stages is trivial to earn, and the SS's were made with the intention of the player getting every single Ring during normal play. You don't even need to be competing at all to go on and get all the Rings, and it's very easy to do normally.
The point isn't simply that they aren't Acts, but that Acts are well-suited for competition by their nature and are always ranked. Those special stages are intentionally simple to max the Rings out on and you're even encouraged to do so naturally, something that doesn't hold true for any of the regular Acts.