I wouldn't call it a loophole. The EU framework does not try to include pricing at all. And indeed the practice of "zero rating" seems increasingly widespread.

It's not a case of opting out: member states would have to make their own, stricter rules if they wanted to ban zero rating.

Surely non-discriminatory pricing of services is a fundamental principle of net neutrality? Without it, you can't really say Europe has net neutrality at all.