I’m pretty sure they didn’t phrase it like that. They probably just asked for a proper imprint with data about the legal representation, which is simply the law here in Germany (see German Telemedia Act) and not something “draconian” that T-Online made up because the don’t “like” certain domains as you phrased it.
But that’s all beside the point anyway, which is that services like United Internet (GMX, Web.de) in Germany or Orange in France have a market share of over 30% in those countries, similar to Gmail in the United States. So blocking these service would be like blocking Gmail.