Thats a great question! I work for a company that considered this technology for our products (not cellphones, but other portable consumer electronics). The reasons HTC and other phone manufacturers don't include inductive charging out of the box are very similar to the issues we ran into (I'll keep the science to a minimum):
-Wireless charging essentially splits your wall charger in 2 parts, and places one half, a "coil", into the device. The other half remains in the "pad". Works sort of like 2 magnets (google 'inductive charging' if you want to learn more). If you've ever opened up a cell phone (espically a smart phone), you know how cramped it is inside there. All the products on the market (ie Powermat) end up adding bulk to the phone. Not alot of bulk, but enough to make manufacturers stop and think because....
-Your average consumer doesn't really care enough about this technology in phones. (I said
most consumers, not people who spend their free time chatting about phones

). Its been around for a LONG time (google Nicoli Tesla). When we did the market research, the majority of people said they would rather have a slim device with a single charger, than a thicker device + wallplug + a charging station. I'm sure everyone here will disagree. But thats just what the numbers said.
If a company can save a few dollars on each device by leaving out this technology, it adds up to millions in the long term. Example: apple has sold 17 million iPhones. If it cost them just $5 to include wireless charging in each phone, you're talking about 85 million dollars for a feature that probably won't grossly effect the sales of the device (i.e, not having wireless charging isn't going to stop anyone from buying an iPhone, AT&T service might haha - but I digress...). The cheap solution is one that Apple came up with for their laptops: magnetic power plugs. I'm guessing most people here have seen a
mac laptop and know what I'm talking about. Its cheap to implement, doesn't add bulk, and makes charging the computer easier.
And now for us, those who love technology: Clearly the demand is large enough for a niche market, and this is why companies like Powermat are selling these devices now. They do want your money!
Send an email to them letting them know you would be interested in a charger case for the Eris. I'm sure there are plenty on these forums that would be interested.