Two things.
1) Let's be honest. For many of us, myself included, the brace was a convenient work around to have an SBR without it being called an SBR or needing to get a tax stamp. I was already on their radar, with a From 4 for a can (which was finally approved between the Form 1's for the two braced pistols that I registered), so there was no down side to getting free SBR approvals.
2) Many owners are woefully ignorant. I recently had a friend show me his braced pistol, about 3 weeks into May. He had no clue that the rule had been processed, and also told me that he couldn't believe that something he just bought would now be illegal. Thus, even though I warned him, he is going to continue to use it as is with no modification.
So, for some of us, the rule is a hidden blessing. For others, it is a hidden trap.