Small startups are often just a handful of people, and devs can and do pull double-duty as QA and development, and sometimes customer support for engineering issues! Freelancers also don't have customer support teams. For larger orgs, however, you're right. And also, like I said in another comment, the "customer" doesn't have to be your customer -- your "customer" could be the QA guy who says he found a bug, or your boss, or another engineer. Whoever tells you they found a bug, you should not dismiss their report with "well, it works for me."

That's what CS engineers and test engineers are for!