I could understand not agreeing with the law but maybe encouraging rogue behavior could come back to bite you in unexpected and deleterious ways
So yes, it's a product choice, but it's not a product choice out of some strange effort for the company to have access to your data, but because it provides the right product mix.
You might as well argue that your bank having access to your account records to hand over is a product choice. Yes, there are ways to work around that. Maybe some blockchain variant will give that level of privacy. It will not do it without significant tradeoffs.
If the private messaging were encrypted by default it would also be easy to offer an opt in feature that would preserve recovery (e.g. by the users software encrypting a copy of their encryption key over to the platform). I expect relatively few users would opt into it.
Imagine that back in the 80s when people regularly communicated by letters in the post if the post office offered an option to open, copy and archive all your letters, just in case you lost them. ... how many people would have opted into it? with the obvious invasion of privacy, vs just taking care to not lose whatever they intended to keep... Not zero, I suppose, but not many. And today these platforms archive and hand over communications far more casual and ephemeral than letters.