I finally found out why Blizzard doesn't allow "recruit-a-friend" in Denmark. I used to think the reasoning was related to multi level marketing, but it turns out that the recruit-a-friend type of marketing ("tip-en-ven" in danish) is regulated by the danish law on marketing ("markedsføringsloven", MFL) and the case UfR 2002.2277/2 SH ("Telia Sport & Fritid") stipulates that the marketing responsible, in this case Blizzard, is liable for the breaches of good marketing practice (in particular breach of MFL § 6, stk. 1 about unsolicited advertisement) committed by the recruiter. I'm fairly sure that's why it's not allowed in Denmark.

I've been wondering about this for a while and wanted to supply the info, should anyone be curious :-)