You mean when you buy a new Mercedes from a dealer is price fixing?
The difference is that Mercedes and most other companies issue an MSRP. Manufacturers Suggested Retail Price. Scubapro, and apparently Aqualung mandate a Required retail price. A practice that in my opinion should be illegal.
Have you ever paid MSRP for a car? I hope not.
I'm not a lawyer, but it seems to me the practice is in violation of the Sherman Antitrust Act. Maybe nobody has bothered to challenge these companies in court. My guess is they aren't treating shops that sell the same product as competitors