Electronic devices of all sorts are allowed on US domestic flights, such as the LGA-DCA-LGA flights you described. There was someone who had their folding key confiscated at DFW last year (a FTer who made the news over that one, IIRC--search this forum and I'm sure you'll find it), but that was an idiot screener and his idiot supervisor backing his idiotic decision that it could be a weapon, and not TSA policy.