You can still do plenty of tinkering with some cars. E.g. you're not supposed to change the battery on the Prius yourself either.
Suffice to say, not all manufacturers are the same and not all products are the same. And frankly, so long as the tinkerers are being serviced, I don't see society as having lost anything yet, just because a single more appliance-y product has hit the market.
And --specifically to the matter at hand-- particularly not when the appliance-y product in question has a first-class, standards-compliant web browser.
Right, but if you were to change the battery in your Prius (and publish instructions on how to do it) you might void your warranty but you're not going to get sued under the DMCA. It seems insane to me that something that could have dangerous side effects (changing a battery in a complex vehicle) is less regulated than code.
By comparison, you're not supposed to change your iPad's battery by yourself.