No, it should be illegal even when done willingly. Because this worsens the bargaining position of everyone else.
I can imagine deals where you get a huge 'rebate' if you permanently enable the ad-feature (the on-screen wizard will blow one of those tiny fuses as its final step, locking the device to that setting). That effectively mandates that the price for the device is its selling price minus the huge rebate, and the whole market will adjust to that.
Just ban advertising on those devices.
> To reserve a Telly, you must agree to use the device as the main TV in your home, constantly keep it connected to the internet, and regularly watch it. If the company finds that you violate these rules, Telly will ask you to return the TV (and charge a $1,000 fee if you don’t send it back).
1: https://www.theverge.com/televisions/777588/telly-tv-hands-o...
The device that immediately springs to mind is the Kindle. You can choose to buy a version without ads, or save ~10% and accept ads.
That seems like a reasonable compromise.