Advertisements are not "free speech"
Although I have no problem with either gun stores, or advertising their products/services, I see no issue here.
Media outlets such as newspapers, magazines, radio and t.v. stations/networks have long refused selling ad space to certain types of products or services...primarily those that anger or alienate their target audience. Like it or not, these are touchy times to showcase anything to do with firearms...that is just the way it is.
You don't see a whole lot of advertising for Nevada cat houses...even in Nevada vacation ads...go figure! (Hey, who knows, maybe they would get more tourists if they did...).
Whether Comcast's policy turns out to be a good or bad decision solely hinges on whether they tick off more folks who want certain goods/services versus those who are "offended" by seeing them advertised. It is always a balancing act.
A good point made by a couple of people here is that if enough Comcast customers were ticked off by the lack of gun shop ads, and subsequently quit using Comcast, then they would probably re-think that policy.
However, I rather doubt that in today's climate that is likely to occur.
Deeds, not words