Yes, this means they can easily rust. it also means--and this is an intentional design feature--that you should be able to UNBOLT each keel bolt, one at a time, and REPLACE IT, one at at time, with new keel bolts. Without dropping the keel, without any fuss. Just a big big wrench and some penetrating fluid needed.
So if someone left it go too long and you try to take them out and something twists off it sounds like a pretty big problem.