I would say as the stuff evaporates (and quite quickly too), your plastics are not exposed to it very long. If you'd put the plastics under prolonged exposure in a can/jar of the stuff I can imagine it indeed can do some serious harm to your items.
Normally I don't use brake cleaner on plastics either, but this weekend I was pretty sloppy changing the oil in my shocks and I degreased the top shock cap by spraying brake cleaner over it. The shock caps didn't die on me (just yet), so afaik all is good.
If you really want to find out what brake cleaner does to plastics, get some old/broken parts of your car, put them in a jar with the brake cleaner and see what happens after a few days.
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