The ads could lead to a cybersquatting dispute.
Elliot Silver wrote about a recent UDRP decision for ter.com. The domain name owner won the case, but the decision was closer than it needed to be.
The Complainant was the French national railway company that runs a service called TER (Transport Express Régional). The domain name resolved to a page with links related to the Complainant and other train transport.
The links likely triggered this case and made it closer than it needed to be.
In most cases, it’s fine to own a three or four letter domain that matches the initialism or acronym of a brand. But you need to be careful how you use it. Parking one of these domains is asking for trouble. Remember that a parking page might show you innocuous ads but show different ads in another geography.
I don’t see much upside to showing ads on one of these domains, but there is a clear downside. Even if you win a UDRP, a company that sees competitors’ ads on a parked page is more likely to file a UDRP, putting you through the expense of defending it.