There is nothing that would stop this legally (what authority should hinder them?). Things that can be enforced are only limited by practicality.

Any non-EU business worth enforcing such a law against will very likely do business inside the EU (e.g. selling ads to local companies). Any money transferred can then be froozen to enforce this law.

Facebook in particular does business as an Irish company for its EU users, so no problem at all here.

So, if you sell products or services in China, you have to comply to all China regulations, not just exports? That doesn't make any sense.

That’s exactly what is happening: Windows 10 "China Edition" or some moves from Apple to comply with Chinese regulations for example.

Respecting local regulations is not a mind blowing fact. A foreign company cannot sell heroin in the USA under the pretext it is legal in their country.

So you mean I can't use <insert shitty country>'s child pornography app in the US?

I know it's an extreme case, but yeah, of course countries expect people who do business in them to respect their laws.

That is the way it currently works, yes. If your foreign web service is not approved by the Chinese government, it will most likely be blocked in China.

Why shouldn't it be valid? As a European accessing US servers I also have to comply to US anti-hacking laws. The only limit is enforceability.

It looks to be the same situation with Apple's Irish Taxes, finally Apple had to comply.