There are limits to what you are allowed to modify on your car. Federal safety regulations require certain features to be installed by the manufacturer, and to be maintained by the owner in a state that they remain functional. Besides obvious things like brakes, you need working headlights, turn signals, bumpers, wipers, etc.
Heated seats is an option. A luxury convenience feature. Most cars don't have heated seats. If you had heated seats, but left them off or the switch broke and they weren't working, there would be no reason for the State Patrol to care one bit that your rump was a bit chilly.
So, the state doesn't care enough make it illegal to have heated seats or not. It is entirely your choice.
The question then becomes, does the state have any reason to care whether you have a manual switch to turn them on or off, or use a special software code to enable the feature? Logic dictates that if they don't care whether or not you have the feature, and don't care if you are using it or not, they would have not reason to care about the particular method you use to turn it on or off...
Therefore the only real question is does BMW care? They might, if you came up with a method of enabling heat without a subscription and it became known to them. Especially if you made money publishing a how-to guide that cost them potential revenue. But that would be a civil, rather than a criminal matter.
To me this action would be equivalent to buying a burger at a place that charges $.25 for a packet of ketchup, and instead using your own ketchup. It's your burger, and your ketchup, do what you want!
Based on discussion on the other answer, as well as a suggestion in comments, I would like to briefly address (my opinion) on the applicability of the Digital Millenium Copyright Act (DCMA) of 1998.
I actually just looked up this act, and have only a layman's understanding of copyright law, but there is a basic element that needs to be met for a violation to occur: The copyrighted material must be reproduced, altered, repurposed, and distributed in some manner. Private avoidance or selective non-use of a digital feature would not seem to rise to that level.
I touched on this above when I alluded to publishing a written hack. It would probably also apply if you offered code that would bypass a feature, or to a car tuner offering to enable the feature for a fee. If there is a commercial benefit, there is a potential "victim", and a copyright issue.
However, even code is questionable... There are many examples of companies offering aftermarket Engine Control Module code to enhance performance or improve gas mileage. Of course a manufacturer could always challenge a commercial competitor, but lawsuits cause money and create publicity - positive and negative. About the only "free" enforcement tool a manufacturer has is to not honor the warranty on any owner altered parts.
Of note is the fact that seat heating elements are not digital, copyrighted, or otherwise protected work. The digital intellectual property that MIGHT potentially be at the center of a copyright controversy is the function of the pay system and the processing of an access code that unlocks a relay. What that relay sends electrical current to is really immaterial. It could be your stereo, it could be the airbag...
One person snipping and splicing wire to avoid the IP "brain" and install a simple on/off switch or rheostat for personal use of seat heat should not be violation of any digital copyright law.