I also use this analogy but I take it a step further to ask this question........... What does buying a property and putting up billboard ads all over the yard do to the property value? And further more what does it do to the overall value of the neighborhood?
To me if you want to treat it like a real-estate then you should be developing the property not whoring it out. So I would say if someone registers www.example.com and parks ads on it they are hurting it's value.... since example.com is an obvious site for the keyword "example" the owner should be placing good content (even if it is static) that drives up the placement in search engines for the keyword "example"
That way when they go to sell the property they can show how they developed it...in the case of domains... show that it already has good rankings for the domain keyword and has no "ad only site" penalties.... someone who just puts click through ad links is nothing more then a slum lord IMO.
So yes it is wrong if they are just gobbling up domains and parking ads and asking you to pay a hugely inflated rate... not legally but in the scheme of good business practices.... If they develop the domain then I think they are doing you a service and therefor asking you to pay a large markup on what they paid is not only fair but is a valuable service as they were working on ranking your keywords before they were even your keywords.