I must say that is an interesting letter indeed!
Although, it is not uncommon to see this. DSL and Cable internet use many different types of technology to reach different kinds of customers. I know that when we finally received DSL in our area, we had to connect to the central office through a remote server in the area, and not directly. This required us to use a different modem than others I knew who used the same service who had the freedom to choose.
While it is a slight inconvenience, especially if you bought the modem you use now separately, this is not something that only Comcast does, and I can't really find it in me to blame them.
You'll have to post the specs on the modem when it comes (model no. and such) as I'm curious to check it out versus the one I have picked out to purchase in case they tell me the same thing!