> The user doesn't get a better price from a dark pool than they would in the market

This is sort of true and sort of not. For an individual trader, it's mostly true.

The law says orders should execute at no worse than the national best bid or offer (NBBO). The NBBO is the best marketed bid and marketed offer. It does not include orders on a dark pool.

A trade executed at NBBO and filled at a better price in a dark pool is perfectly legal and somewhat commonplace. This is what Citadel et al do.