For the same reason that a person under British jurisdiction is subject to its laws. I mean, laws, including British ones, exist for a reason. And usually, that reason isn't "because they're British," and this reason makes sense regarding to non-residents as much as it does to residents.
> Just because it's accessible globally means I now have to factor in every possible regulation from around the world?
Literally. Unfortunately, international laws and international cooperation are not yet sufficiently developed, and extradition requests for such reasons are not a common occurrence yet.
> The burden shouldn't fall on me
The burden doesn't fall on you. That's not how laws work. Laws usually work like this: The British government decides that certain actions are harmful to the UK, and to prevent them, it punishes those who commit them. So, on what basis should you be granted an exception to this logic? These prohibited actions don't become any less harmful to the UK just because they were committed by someone in another jurisdiction.