It's only irrational if you think the government cares about maximizing hashing efficiency. It's perfectly rational if you recognize that in most (western) legal systems there is a difference in the ease and acceptability of stopping someone from taking a new action and the allowing them to continue taking that same action. This is due to various legal underpinnings, but also makes perfect sense in that allowing someone to invest millions and then pulling the rug out from under them is very different from telling them they were never allowed to invest those millions in the first place.

Building codes are an excellent example of this. It's perfectly legal to maintain, sell, lease, etc. a building that would be illegal to build today.