In December, the Dutch Authority for Consumer and Market ordered T-Mobile to stop offering its so-called zero data plans, which allowed customers to stream music from participating apps and services on their phones without counting against monthly data allowances. But the Court of Rotterdam ruled last week that the program did not violate net neutrality rules, the NLTimes web site reported.

According to the court, the European Union’s net neutrality rules, intended to prevent carriers from discriminating against online content and services, allow the zero rating plans even if stricter rules in The Netherlands forbid it. T-Mobile has said it was not discriminating because any music streaming service could participate without charge.

Regulators across the world have moved to ensure that net neutrality will be protected, but enforcing the rules has proven complicated in the case of streaming services that don’t count against monthly data limits. In the United States last year, the Federal Communications Commission okayed a similar T-Mobile (TMUS) offering but moved to stop AT&T (T) and Verizon (VZ) from favoring only streaming services they owned with exemptions from data allowances. However, after President Donald Trump appointed a new head of the agency, the FCC dropped the efforts in February.

Get Data Sheet, Fortune‘s technology newsletter.

The problem is that while net neutrality rules were designed to stop carriers and other Internet service providers from blatant discrimination, like blocking web site access from their customers or charging extra fees for some services, the zero rating plans all provide some extra benefit to customers. Regulators have been trying to strike a balancing act, allowing some pro-consumer practices but stopping the kinds of favoritism that might hurt competition.

After last week’s court ruling in Europe, the Dutch Authority said it was pleased that the decision added clarity to the net neutrality rules, but was uncertain whether it would appeal, a spokesman said, according to the NLTimes.

“The ACM has another six weeks to decide whether to do so or not,” the spokesman said. “We will first study the ruling carefully.”