If a robot makes a meaningful contribution to your website / service, do you really care if they are a robot?
If you could tell which comments are insightful / relevant / interesting / unique it wouldn't matter which ones were produced by humans and which ones by algorithms.
Likewise humans can often create spam by hand - daft comments / contributions that hurt your site but come from legitimate humans.
The assumption with any kind of collaborative filtering is that the opinions of many people produce a better result when combined than the opinions of one person. If you are allowing machines to vote then you're letting one person have an arbitrary number of votes, which totally breaks the model.
If you could tell which comments are insightful / relevant / interesting / unique it wouldn't matter which ones were produced by humans and which ones by algorithms.
Likewise humans can often create spam by hand - daft comments / contributions that hurt your site but come from legitimate humans.