And what law prevents a Russian to buy bitcoins on a foreign exchange and keep them in a wallet abroad?
At least, so far there was no statement from Russian authorities on the legality of Bitcoin ownership per se. In practical terms, any attempt to police Bitcoin ownership by individual users will not work. Also, there is no law prohibiting such ownership that I'm aware of.
However, Russian police is not very concerned about following the letter of the law. They routinely miraculously find "just enough crack to put in jail" while searching any person those in power don't like. If they find bitcoins while searching the house of an opposition figure, for example, they may throw a "money laundering using cryptocurrencies" charge on top of all the others, and pliant Russian court will just rubber-stamp the charges as it always does.