They can need protecting, depending on the environment, how they are carried, or how they are stored and for how long. I have had rods corrode while sitting in a wooden drawer in my house. My friend who does jungle trips says that they all but dissolve in that environment, unless stored in a sealed container.
The corrosion isn't rust, it is pitting with a grey powder residue. They definitely corrode more than plain magnesium.
I see someone thinking that "cheap and cheerful" might be linked to corrosion. This isn't right. The reactive ones corrode. These are the ones that are easy to get a spark from. The ones that you can scrape and scrape and scrape with ever increasing pressure until you eventually coax out a massive hot spark (and scatter your tinder) are the ones that are less likely to corrode, and those tend to be the cheaper ones. Cheaper because they have less of the rarer, harder to work with, more reactive metals in the alloy.
I use clear nail varnish on mine between trips, but then I am not carrying mine all that often.