There are plenty of reasons why someone might do things that you do not expect them to do. That you seem to believe there is a "one size fits all" approach to TM enforcement indicates you have not seen many situations.
There are, for example, certain companies which are perfectly aware that there are hundreds or thousands of variations on multiple trademarks of theirs which are registered. They do not want to register all of those domain names, nor do they want to pay $1500 a pop for UDRP proceedings on all of them. If they simply do not want those domain names used for PPC advertising, etc., then paying $.37 for sending essentially the same form letter to hosting companies, which are typically highly risk averse and compliant, is tremendously more efficient.
Absolutely, the domain registrant can have that domain name pointed somewhere else in 24 hours. And if the TM owner sends 365 letters per year to different hosting companies, then the total annual cost is still only $135 for aggravating the heck out of the domain registrant. Even if the TM owner would prefer to obtain the domain name, after enough aggravation, the domain registrant may not renew, and if it is a not particularly attractive variation, then they may pick it up in a wide net of snapbacks they have for the hundreds of domain names on their list.
To the domain registrant using Sedo parking, the perspective is one of a single domain name and a single trademark. To the TM owner, they are often looking at a much larger picture, and using a broader strategy to pick their targets for other types of enforcement actions.
The notion that "you only contact upstreams as a bluff" is a remarkably limited view. If a client walks into your office saying, "I have a trademark that is being used in 500 domain names. Some of the domain names don't point to anything, some point to porn, some point to pay-per-click parking pages, and some of them are used to sell competing products.", then what do you say?
"Oh, no problem, just give me $750,000 and we'll file 500 UDRP cases."
Yah, right. As they walk out of your office, could you please give them my number?