Under German law you are subject to German laws about commerce if you are "addressing the German market". Some indicators include offering your site in German, probably also support German phone numbers and addresses, etc. For some services/apps this is obvious (e.g. online shopping) for others it is more debatable.
The most important part however is that of course you don't have to give a damn about what German law thinks as long as you aren't in a position in which German jurisdiction can be enforced. Likewise, even if you are a German citizen living in Germany some German laws and regulations may not apply if you are decidedly not offering your services/apps to a German audience -- though of course that's a much less safe position. Either way, it's not as simple as "it exists on the Internet, therefore it falls under German jurisdiction" although the reasoning is quite similar to that in Brazil.
The point is moot, anyway. Brazil can't enforce their laws against a US company that doesn't have a presence in Brazil, but it can ban them from Brazil -- as apparently a Brazilian court is allowed to force Internet access providers to ban specific IPs. Whether courts should be allowed to do that is a legitimate question but right now in Brazil they apparently are, so everything is fine.
This isn't an action against WhatsApp. This is an action against Brazilian WhatsApp users. It's basically enforcing a sanction against WhatsApp by preventing Brazilians from accessing the service (which they can't get at otherwise). This is more like a German court forcing an IP ban (in Germany) against a Nazi website hosted outside of Germany -- which is a thing.
The most important part however is that of course you don't have to give a damn about what German law thinks as long as you aren't in a position in which German jurisdiction can be enforced. Likewise, even if you are a German citizen living in Germany some German laws and regulations may not apply if you are decidedly not offering your services/apps to a German audience -- though of course that's a much less safe position. Either way, it's not as simple as "it exists on the Internet, therefore it falls under German jurisdiction" although the reasoning is quite similar to that in Brazil.
The point is moot, anyway. Brazil can't enforce their laws against a US company that doesn't have a presence in Brazil, but it can ban them from Brazil -- as apparently a Brazilian court is allowed to force Internet access providers to ban specific IPs. Whether courts should be allowed to do that is a legitimate question but right now in Brazil they apparently are, so everything is fine.
This isn't an action against WhatsApp. This is an action against Brazilian WhatsApp users. It's basically enforcing a sanction against WhatsApp by preventing Brazilians from accessing the service (which they can't get at otherwise). This is more like a German court forcing an IP ban (in Germany) against a Nazi website hosted outside of Germany -- which is a thing.