There is good reasons for this, it can help a ton in figuring out where exactly the issue is. If the speedtest server is off-net, you have no idea where the problem lies. Transit provider? Peering? End network? my wifi? Last mile?

There may be some good reasons for this, but for even the best companies, it's shady. And comcast is far from one the best companies.

Instead, they should create a site that is clearly comcast branded and shows the results of each step.

They do: http://speedtest.xfinity.com/