The problem there is, if Google donated $7.5 million (15,000 engineers * $500) to the Eclipse Foundation, that would certainly give the impression that Google had an unfair amount of influence on the direction of project, compared to a small company only paying a couple of hundred $ (or an open source developer, not paying anything at all).

I think that goes directly against what you say is the primary benefit of open source: that no one company has an undue amount of influence on the product.

I think it's much better that they contribute a few engineering resources directly to the product, rather than just handing over money. That way, they're guaranteed to be engaging directly in the open source "process".