These ones aren't smiling at you. They're smiling at blini and caviar.
In Russia, a polite smile is seen as artificial and strange. Russian etiquette would rather you keep a serious, even grim face, to the point where tourists think the whole nation is suffering from a serious case of Resting Bitch Face. It even extends to customer service: no "smiles are free so give one to every customer" crap here. The only time your waiter is supposed to smile is if they know you; flashing a polite grin at strangers is generally completely out of the question.
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If your waiter doesn't glare, he's probably an American spy.
Russians tend to associate easy smiles with negative features like insincerity, secretiveness, and unwillingness to communicate true feelings. Smiling by default is seen as a lie, as befits a country where vodka poisoning and bear maulings are considered wholesome entertainment. You actually need a reason to smile in Russia. Acceptable excuses for baring your teeth include seeing good friends, hearing a great joke, or, of course, if you're about to bite somebody.