A good point.
In fact, I'd almost prefer a comment service start serving up porn on my blog. At least it should, presumably, be fairly clear that it's spam, and not something I've actually endorsed. This "recommendations" feature is essentially Disqus abusing the fact that they can generate content on my site to do one better than the comment spammers they protect us from, and make it look like I'm endorsing their random links. Spammers' wet dream, really (the fact that they're links to well-known publications and not virus-laden sites notwithstanding).
I disagree with this. There is a moral obligation on both parties regardless of the presence of a consideration (payment of money) in the contract.
There is a binding contract, albeit most likely not legally enforceable and of uncertain terms. To consider an extreme, what if disqus has said they were going to include porn links in their plugin area. Some might well consider covert advertising as more insidious than inappropriate porn.
If we're in a bar and you need to use the WC, I offer to watch your stuff but instead I just walk off as soon as you leave the room. Well, "no binding contract" but I've still acted immorally and anti-socially.
Offering someone something free-gratis doesn't give you carte blanche to screw them over as soon as their figurative back is turned.
tl;dr I offer you a free beer, ha-ha I pissed in it, "what's the big deal ? did you pay for it ?"