This already has been investigated by @smartsteem and it seems like those guys who send in more SBD than the bot gives out are abusing "front runners". Essentially, some upvote bots have this group of people who would upvote the post before the bot itself upvotes. These "front runners" for that particular bot has a combined vote of $50 and they would vote based of detection SBD transfer to the bot. So the step to abuse it is pretty simple:
- Send in more money than the bot can provide
- The bot refunds your money
- Get $50 upvote without actually spending any SBD
- Transfer the same amount of SBD to your friends and repeat
The negative return is actually not real since the bot did not vote, therefore its voting power remains untouched. It's because Yabapmatt has to do a history grab on the blockchain instead of using a provided API from the bot. This creates inaccurate estimation of ROI. Yabapmatt explained this himself so whoever votes during that time period actually makes more money because more people back off out of that time frame.