This is a key point. When level capped content in CoP was introduced, there were already capped BCNMs in the game... they knew how to keep characters under cap. There was also no possibility of changing jobs in the middle of the fight, so the cap only needed to be applied and calculated upon entering a capped zone. Applying a cap is a little more complicated in FFXIV since we can change classes freely ( and I'm guessing they don't want to disable this ability ) and the level cap would have to be checked every time we did this.
On the other hand, a one-time difficulty selection upon entering is very manageable, as it is already in the game for levequests.
My preferred approach though, is to include high-level passages and rooms. Just because the main content is geared for 25-30 doesn't mean there can't be deeper, more dangerous areas that provide a greater challenge and greater rewards.