Yes, I strongly advise reading the TOS (Terms Of Service) from any parking service that you are thinking of using and then also CONTACT them to make sure. Sometimes the TOS will change but not get updated or just be confusing.
As the others have said, most parking services only allow type-in traffic and traffic that comes from old search engine listings. There is at least one that does allow you to get links from other sites as long as they are the normal kind. And I hear there are some parking services that allow you to use Pay Per Click advertising to get visitors to your domains, but you really have to know what you are doing to make any money at that.