The jeweler Tiffany® may seem like one of those companies that regularly pops up in the news suing companies left and right for trademark infringement and related issues, but Tiffany® is not alone. Other companies are notorious for going after alleged trademark infringers. (Think Mattel® and all the litigation you've heard about over the years related to Barbie®.)
Sometimes it seems a company will sue for anything, but did you know there's actually an explanation rooted in the law rather than simply a desire to crush the competition? The US Patent and Trademark Office (USPTO) does not pursue alleged infringers on behalf of trademark registrants, and failure to police one's own mark could, in a worst case scenario, result in the registrant losing its trademark. That's because a failure to make others aware that you own the mark (i.e., a failure to police your mark) may be evidence that you've abandoned your mark. (Refer to footnote three on page three of that last link for an example.)
Unfortunately, aggressively policing one's mark can result in pursuing folks who aren't infringing at all, potentially stepping on their free speech rights. This comes up often in cases where defendants are artists alleging fair use (see bullet point 10 for an example).
Fair use is not a likely defense for Costco® in this case, however, but you might be interested in following the case to watch for Costco's next move. For more on the Tiffany® v. Costco® case, check out Bloomberg Law. (Make sure you're logged in before clicking the link.)
For more on trademark law, head to the third floor of the library, turn right at the top of the stairs, and check out the books that start with the call number KF 3180. You can also go to the reserve room behind the circulation desk and find trademark law treatises and study aids around the call number KF 3180.