That seems like a weak argument, but the courts make all sorts of surprising decisions these days.

In the past, Nintendo tried using their trademark as a watchdog -- the console would reboot if the game did not display it at startup -- then sued unlicensed games for trademark violation. The courts ruled the trademark was purely functional in that use case, and not a trademark, then ruled against Nintendo.

I imagine a similar ruling, if the defense is: The user has a valid key, and a copy of the media. Any circumvention happened before that point in time. We're simply building a compatible device.

Of course, I'm not a lawyer, and the DMCA is a terribly designed law.

> In the past, Nintendo tried using their trademark as a watchdog

That was Sega v Accolade [1]. Although Nintendo also uses boot time trademarks, I don't think they sued over them.

[1] https://en.wikipedia.org/wiki/Sega_v._Accolade

That case was before DMCA made it generally illegal to break any copyright protection measure.

It also wasn't protecting the games from being copied. It was protecting Nintendo (actually Sega) from competition from other people making their own games. The Nintendo Game Boy uses the same technique.