The legality is a very non-subject, especially considering NEC abandoned the scene years ago and no longer cares...of course, pirating games is a subject left to the individual companies that own the copyrights but the hardware itself is of no real concern to us in terms of it being legal or not.
You're right - I never thought about it that way. If the Flash Card was sold with ROMs on it, then of course it would be illegal, but on its own it isn't.
Actually, it depends on what country you live in, since copyright and IP laws vary greatly. In some countries/territories (i.e. Hong Kong), rather than going after the folks who own pirated content (ROMS, movies, music), they go after the "bigger fish" -- namely, the folks who produce / distribute hardware that "faciliates" pirating ("sharing") of copyrighted material.
Thankfully, here in the States, IIRC, the law acknowledges that hardware can have legitimate uses (i.e. developing new games). 99% of the folks who get flash carts use them for pirated ROMS, of course, but at least U.S. law doesn't criminalize the hardware.
Correct me if I am mistaken on any of these points.
As folks here know, I prefer real hardware and only play games I own (well, 99.99% of the time). That said, the prospect of playing ROMS on real hardware is very tempting. I can see the appeal.
Thankfully, I own so many games that I've barely sunken my teeth into, my gameplaying schedule is solidly booked for the next few years (decade?) ... no moral dilemmas for me!