I assume that this is to prevent people from making an infinite number of trial accounts and never paying. If you're on Windows, Netflix will use Silverlight to fingerprint your system too. On Android, they will grab the device ID. Not sure what they do on iOS, since I don't use iOS.
They use your credit card to ensure you don't get 2 trials. Once you run out of credit cards, you have to pay.
Isn't EME also disabled in incognito?
and the easier way to prevent that would be to require a credit card wouldn't it?
This is not new. They've been doing it for at least a year.