I think I mentioned this waaaaaaaaaaaaaaaaaaaaaaaay back when the game first came out :P

It should be pointed out that not ALL streltsy were equipped with muskets, or, indeed, ranged weapons. Some were equivalent to policemen or a local fire squad (as mentioned in the flavor text).

Still, it is somewhat strange that they're equipped with bardiches, which were traditionally used by old style musketeers as a brace for firing the musket from, back when they had to be made with inch thick barrels and weighed a hundred pounds. Purely melee streltsy would probably use swords or something.