Contrary to popular myth, stainless steel (or, corrosion resistant steel for the pedantic) does have a magnetic attraction. It is much weaker than high carbon for sure, but it is there nonetheless.
It's actually the amount of nickle in the mix that gives it, it's corrosion resistant properties. A metallurgist could give you a difinitive answer, but I always believed the attraction was in relation to how much iron the nickle displaced. More nickle meant less iron for the magnet to work on.
As always, the deeper you look, the more complicated it becomes.
I say go ahead and use the cables. Stainless cables, in my experience, last a lot longer & perform better than the zinc plated varieties.
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I shouldn't have to "make myself more visible;" Drivers should just stop running people over.
Car dependency is a tax.