Having a law in place is a start, but it has little effect against those who would cut corners. I believe the saying is that "laws only keep the lawful honest." There will be a lot of manufacturers building TacoCopters who will cut corners because it's cheaper, and others who simply don't care because the don't expect to get caught.
However, if the FAA mandates (e.g.,) complete engine teardown and rebuild every 200 operating hours along with an annual FAA inspection, they can actually verify that those procedures are being followed. The law will still be in effect, but verifying that it is being followed happens well before someone is killed and will take the vehicles most likely to kill someone out of the air.
That's what they're trying to accomplish.