Yes that's on purpose. The idea is that we share the load of guaranteeing so that if a spammer is discovered it affects less on the chain.

I believe the 20 instance limit is intentional (its a max/permanent limit, rather than daily), as its meant to be a "Pay it forward" type of deal (and I imagine maybe to prevent one instance from going crazy with it). Don't quote me on that though, as I'm not the dev of the project - db0 could probably clarify this though!

That being said, while you can't guarantee more instances after you've hit 20, the instances that you've guaranteed can go out and guarantee more instances. So you can always go ask some of the instances that you've guaranteed to evaluate the instances that you want to guarantee when they have time available, and see if they agree and will pass it along.