Generally speaking, I think the flags against those using bots are another form of "disagreeing with rewards" which is the number one reason to downvote something. The whole point of upvoting/downvoting after all is to influence how rewards are distributed. More rewards for bot operators and those who use them mean less for everyone else. In the case of potato/burn posts, it is perceived that those are helping the value of steem and/or SBD and are therefor helping everyone or at the very least that the rewards are not being used for selfish purposes. Also (AFAIK), those votes aren't bought. In the case of those that buy votes for profit, it is different. I would think the reasons would be obvious.