I have never had any problems with Tmobile about getting a phone fixed. But i have a question. Did you have insurance on the phone. A warranty is different then insurance. With tmobile insurance you send the phone in and they either fix it or give you a new one. You said warranty so i assume that you are trying to get it fixed with a HTC company warranty. HTC can reserve the right to not honor a warranty if they think it was your fault.( they do not have to have proof)
The point of phone insurance is that Tmobile provides the replacement and then bills HTC. Alot easier then going through HTC. IF you had insurance with tmobile they should honor it. try calling customer care a few times. Sometimes you get a new person that doesnt know or is not sure. IF you didnt have insurance through Tmobile it is not ripping you off. You ripped yourself off.
Also usually your phone insurance has a deductible. Just like car insurance. u said they were going to charge you 100 dollars to fix it. that sounds like your deductible to pay on that particular phone. Most likely they would send you a new one. So you pay your 100 dollar deductible and get a new 500 dollar phone and they eat the cost of 400 dollars sounds fair to me. Now again thats with insurance. sounds like you didnt have any but i could be wrong