No services really requires data transmission. It's just the owners have decided to run it that way.
> I expect users will check the checkbox without reading any explanations, just as today they click "I Agree" without reading anything.
Within EU Data Protection law, you have to get "freely given, informed consent" for things. And a checkbox which links to 30 pages of legalese does not necessarily count as informed consent. Just because your users clicked a checkbox/clickthrough, that doesn't mean you're in the clear legally.
> if this is a legal requirement, then service providers are greatly incentivized to require data transmission as a condition of using the service
Unrestricted data transmission (and processing) is not legal in the EU. EU fundamental rights law specifies that. So you can't necessarily create a service which from the start requires that sort of agreement.