At some point this incentive mechanism would stop working because a party receiving 0 vs a party receiving non-zero would benefit more from having money at the margin.
I'm not sure I'm seeing how "most ads aren’t for getting people to switch sides" means "$10m of ads for one party and $8m in ads for the other is not equivalent to $2m in just one"
Maybe you would need a trust in between?
But yeah, good point, it might explain (more) why it didn't blow up.