If the physical item needs an ongoing, active service in order to do the things you're paying for then that's understandable - SatNavs need data connection for corrected maps and live traffic. Phones need data for internet and calls. Home broadband needs an ISP to P the IS. Sky TV is a service provided to your SKYbox thing, and so on...
But if the thing can simply work fine without needing such a service, then forget it. There's no need for ongoing charges.
If there's a special unlock of an advanced mode, maybe charge a one-off supplement fee for unlocking it, but other than that it's just blatantly milking the buyer and really not acceptable.