I noticed the same thing at Back Bay Station. Then a few weeks ago they tagged a bunch of bikes with warnings that they'd be removed if not claimed. Then they still sat there for a while. I think they finally got picked up now.

I agree the best choice is to notify the police (or security if it's private) - the cops can come by and put on the warnings and then eventually remove the bikes. Although.. I guess the police have better things to do.. but still - doesn't do much good to have a rack that is full of rusty old bikes either.