My guess as to the dynamic is this. If a VC knows that SOME other VC is interested in you, they can't feel out other VCs to see who it is because it might get a third VC involved.
But if one VC knows who the other VC is, then collusion is simple and clearly in the VC's best interest.
No, again it isn't. If you are looking to raise $1M on a $3M pre and you're talking to two firms seriously with the idea of picking one investor (who would end up with 25% in that scenario), you might negotiate some of that point, but its pretty unlikely you will end up taking 1 on 1 ( with the investors each putting in $500K, which would get them to that same 25% ownership threshold as in the original scenario).
But if one VC knows who the other VC is, then collusion is simple and clearly in the VC's best interest.