German law basically says that terms of service and end user license agreements can't contain surprises. If a service explicitly states it doesn't do certain things, the user hasn't consented to the service doing those things, even if the ToS or EULA contain a snippet in fine print saying the service may actually do these things after all.
This already bit WhatsApp when they tried to ban users for using alternative clients. A German court ruled while they may block those alternative clients, banning the user is unexpected because most other services don't have that kind of policy on alternative clients.
So they could have still kept that policy but they would have to make it unambiguously clear and draw attention to it outside the ToS or EULA because it's something users wouldn't expect from that kind of service.
