There are always underlying reasons to ban or legalize something, but I fail to understand what Putin is gaining by legalizing Bitcoin. Maybe Putin is cool all of a sudden? doubt so.
One of the most useful utility that Bitcoin has is as a 'wealth pump' of sorts. Achieving this with more mainstream systems is fraught with difficulty...particularly for Americans. Perhaps Russia is anticipating a scenario where the direction of flow will be favorable to them. Just a guess.