Rust? Make em out of a metal that doesn't.
If it's a big deal then surely it's not beyond the wit of man to build a cover into the bike design? Outdoor mains electrics have them.
I can see that you are not an engineer!
Unfortunately no one metal or alloy is ideal for these contacts, any one chosen is a compromise beteen the different properties necessary to do the job, and all the most suitable ones suffer from atmospheric corrosion and contamination in the presence of water and air to some degree. In addition rain water is not always clean and the contaminants can be conductive, so an accumulation on the battery mounting platform can lead to current leakage across the two terminals.
A retracting cover to conceal the protuding prongs would be bulky and need the bike frame size to be extended somewhat, a very undesirable thing in a market where many already don't like the length of integrated battery bikes. It woud be possible of course, but would add to the cost and complexity for everyone, simply for the sake of the very small minority who remove the battery with the bike exposed.
Panasonic have been producing some of the best units around for a decade now and as the largest company in the world in consumer electronics could hardly be accused of not knowing their stuff in electrical design. Here's the excellent shielding of the battery terminals on their unit:
And a pair of the three terminal posts that stick up from the bike: