None of the domains I own are trademarks and I've owned them all for 2 years or more. I'm past the blackmailing stage!
There are varied reasons. e.g on one site, I can see in the web archive that they were a small commercial business, yet they have links from some amazing sources that would normally not expect. I'd love to ask them about that. Another domain, is still obtaining new backlinks, even though I haven't put a website up or done any SEO work. I'd like to speak to the previous owners and see if they are doing something.
Stuff like that. With over 1000 domains in my portfolio and 100+ websites, there are going to be a few odd ones in there. I just don't want to give $500 to domain tools for a few email addresses if there is an alternative out there for free/cheaper.