Not screening spam content isn't a big problem, it's just a matter of time before buying a vote cost most than the reward it can earn you.
The problem with bidbots is those who don't have any max cost protection.
If for example the weekly cost average is 2% for a vote and two people send a bid for 90% of capacity at almost the same time it will incur a 60% lost for both while it was clearly not what anybody sane had in mind.
The cost of the vote is variable and usually dependent on demand at that particular time, no?
The big bid-bots have protection from max downside loss. Smartsteem and The Rising for instance have 10% max loss on each voting round.