I've experienced stainless a fair bit at work, there are actually many grades of stainless available, some are magnetic (austenitic type), some are not, some grades are more resistant to corrosion than others, some machine better than others, some work harden quicker, my guess is that the spokes were made in a very low grade of stainless and will therefore rust fairly quickly in the presence of ordinary water, let alone water that contains road salt!
As an aside a bit of an "Achilles heel" with stainless is that most grades suffer from "stress corrosion" in the presence of chlorides eg salt, invisible cracks develop in certain ciircumstances, so for this reason alone one has to be careful to always select the best grade of stainless for a given application.
I don't know enough to say what the correct grade of stainless would be for spokes
but high on the list of requirements would be strength.
