Comcast completely denies their claims. In a blog post, the company said "We have no policy against Tor, or any other browser or software. Customers are free to use their Xfinity Internet service to visit any website, use any app, and so forth."
According to a report on Deepdotweb, Comcast customer representatives have branded Tor "illegal" and told customers that using it is against the company's policies.
Tor is a type of web browser that, in theory, makes all your internet activity private. The software routes traffic through a series of other connected internet users, making it difficult for governments and private companies to monitor your internet usage. Up to 1.2 million people use the browser, which became especially popular after Edward Snowden leaked information showing that the NSA was eavesdropping on ordinary citizens. Prior to that, Tor had been popular among people transacting business on Silk Road, the online market for drugs and hitmen.