Because Roku is a one-time purchase, where additional cable boxes are monthly fees. I already own the Roku. By taking away HBO Go access on it, I can no longer watch HBO in that room at all.

Last I checked, all the content wasn't available on-demand either. There was a larger selection on HBO Go versus On Demand. OD only has the last 1-2 seasons of the series it has at all, and has fewer movies. HBO Go has all the seasons of all the series.

My iPad was a one-time purchase and Comcast doesn't have a problem with me streaming HBO Go content on that. So why the Roku?

Is your iPad permanently hooked up to your TV? Many people have rather large HDTVs and would much prefer to watch on there rather than on a tiny screen you have to carry around or dock somewhere.

Comcast doesn't sale or rent iPad competitors so they don't feel the need to cripple the iPad.

Maybe your question is genuine, so I'll reply with my genuine response: why not? What business is it of Comcast if someone wants to watch an Internet service they've paid for on their Roku or Playstation or Linux PC?

Xfinity On Demand's user interface is atrocious.

HBO Go has a much larger catalog than Comcast on demand. You can also stream in HD, even if you don't have Comcast HD.

This is a strange argument. What if you don't actually subscribe to HBO, but still have account privileges?