The submitter says "be careful when you exercise your first amendment rights," which attempts to frame the issue as one of "free speech."
Really, it sounds like the guy called up Comcast, was a total asshole, bad enough that a guy at Comcast told his employer what kind of person they kept employed. Bad enough that his employer would fire him for it, so we can only guess at the content, but I'm willing to bet it was pretty abusive. Those customer service people put up with a whole hell of a lot on a daily basis, so this was probably something above and beyond the normal abuse people hurl at Comcast (justly or unjustly).
You could argue that the employer should have shown the guy the email summary, but that's on the employer's conscience.
Like, I know that Comcast is a terrible company, and it sounds like he was right to be pretty upset with them for the terrible customer service he received. But given that he makes no attempt to explain or defend what he said on those calls, I'm guessing he crossed *way* over the line. If you're a terrible person, maybe you should be fired.