Yeah that is kind of stupid. Did you try to ask for a supervisor? If I'm not mistaken if it just feel out it will be covered under the factory warranty for workmanship. Try calling HTC about it, they may replace it for you.
if you have it, then just stick some thin double sided take in there like it was, if you lost it contact HTC and see if you can get a replacement lens.
That sounds like a load of bull**** from verizon. The phone should have a 1 year manufacturers warranty. So they should replace it no questions asked. Id raise some hell.
Glad you were able to fix it! Out of curiosity was it glass or plastic?
EDIT: To add to your original post, take it to a VZW store and have a rep notate your account if it happens again. This kind of stuff IS covered, I worked as a Supervisor in a corporate store for a while. (taking care of customers is #1 at VZW)