I'd have to see it before rending an opinion, but there are plenty of ways for water to get into a bearing, even if you never rode in the rain. After all you probably rode through puddles at one time or another. If water is in the BB shell (behind the bearing, or outside a cartridge BB) it might have entered through the seatpost, or more specifically the pinch bolt area, and run down into the BB. This is the most common point of entry. If the water entered from the outside, under the seals, that's from wheel spray directed at the gap between the cranks and BB.

It happens, but given the age of the bike, you might be entitled to some consideration if the rust is enough to have damaged the bearing already, even if it's only an act of goodwill from the seller.