In general, it is LEGAL to chum. You can chum with corn, for example, to bring in carp. I know people that chum with tiny bits of chopped up cooked spaghetti in deep water for burbot and whitefish. So, in general, there are no laws against the act of chumming itself.
However, if you were to chum with minnows, they would have to be from an approved source. Imported salted minnows would make a good chum, provided they follow the guidelines on imported preserved baitfish (check the law book). Or you could just kill leftover bait and dump it down the hole - that's basically chumming as well, and would be legal.