Don't jump to conclusions here. It's not like IDW isn't paying the creators they hire to do new work. These are reprints, however, that were commissioned and published by other companies, and which IDW bought the rights to reprint from CBS/Paramount. The legalities of the deal are no doubt complex (especially since the company that commissioned the DS9 comics specifically under discussion no longer exists), and there may be more going on here than meets the eye, especially since IDW is a relatively small company.
I'm not speaking with any insider knowledge here -- although my dealings with IDW as far as prompt payment for services rendered have been excellent -- but my years in The Biz tell me that when you're dealing with this much company-hopping and rights swapping there are many intricacies to the deals, and it's very rarely ever as simple as "they're not paying because they're bastards."