Hi
Yes the use of Bid Bots is allowed on Steemit. It is an organized activity, which Steemit witnesses participate in by owning Bots. It is controversial and one reason it is discouraged is that some Steemians feel you should get all your rewards from the upvotes of your followers. I am sure there are other reasons such as the risk of losing money and giving money to Steemians who create Bots but not content. There are other issues with posts which are considered low quality making it to the top of the trending page not based on the post’s merits, but on the amount of Bid Bots used. There are other perceived negatives also. This is the blockchain so there are very few rules, but many opinions.