In general, nothing stops it, although it's not super difficult for me to manually discount such votes, so it would result in people wasting their money on the bid bot. But, you're correct, as a general mechanism for making stake-weighted decisions it suffers from a lot of problems (probably the most serious is that delegations change the result vs the true stake-weighted vote which determines witness).

I mostly ran this poll to see if there was a "clear winner", but it seems there wasn't.

So the only way to resolve the issue appears to build the proposal system first, then use it to make true stake-weighted decisions about funding sources.

As a side note of possible interest, I believe BitShares has used their proposal system to run polls where the proposal didn't involve any real monetary awards, but where it was instead used just for stake-weighted polling purposes only.