Many ppl do have channels/networks/websites in their signatures. Yes, you could broadly interpret that as spam, however since there are some valid reasons for including them, we've been in the habit of allowing that. Of course, some ppl take way too much liberty with that and the best we can do is try to look at the intent as well as the nature of the website/channel. People can include such info in their profile and so far abuse of giving some leeway in sigs hasnt been rampant.
In the case of channel/network, it does enable a poster to contact another person for clarification of a post or for further discussion rather than waiting for a reply here.
Basically, posts which invite users to channels/networks/websites, potentially harmful sites, nonsense about get rich quick schemes, warez, hacking etc arent going to be allowed anywhere. Often ppl will post helpful websites where ppl can get legit info on a question, so again its a matter of intent.