If you wanna go by strict law then nothing illegal has happened
1. 17 years is a legal age in Russia in special cases (I believe in some Russian republics minimal age is actually 14 years). It is a special case, because parents agree.
2. Russia wouldn't register polygamy marriages, AFAIK it didn't happen on paper, because he was married unofficially with his first wife (thus not married by Russian law)
3. Age difference is not illegal
So what we have as a reaction in media is "unacceptable customs" not laws.