Your explanation describes exactly how it is not a "fair" business model. The upvote bot is performing a service (upvote) at no cost to themselves and providing a measley amount of consideration. The only way you get ROI when using these bots is if the value of Steem appreciates enough by the time your 7-day payout period ends. In many cases, you will lose more because of the depreciation in the value of Steem.