I'll add another argument that doesn't depend on voluntary goodness, just on longer-term thinking: if you can establish a reputation that you're not making it hard for users to migrate away from you, people will be more likely to try your services out in the first place.
And yet another long-term thinking argument: if being able to easily export and migrate data between competing services becomes commonplace, then you'll not only have an outflow of users to competitors, you'll also have an influx of users migrating from your competitors. If you're trying to put a superior service on a market dominated by inferior incumbents, it's in your interest to promote data portability, as - if your service is truly as good as you think - user flow will predominantly go towards your business.
Is the point that you should feel good about any regulation, by virtue of its being the result of a democratic process? For (counter)example, I might not normally feel good about implementing “Muslim ban” functionality even if I recognize the nominally democratic process that forces me to.
An answer to "why" that does not depend on voluntary goodness is: Enough people is the world generally think representative democracy is a good thing. We stand by that system for making the rules. Enough people in part of the world think there is enough of a problem to the point where a rule was made. If you want to do business in that part of the world, you need to be bound by that rule. That's why you should spend time on it. As incomprehensible as it might be, it's important enough to those citizens that they are willing to levy a penalty on you if you don't.
...and so far, at least three people actually took time out of their day to go find the "down" arrow on this obviously raving insane viewpoint :) I love you guys!