I don't see any problem with companies sponsoring tags that are products/proper names, like flex or flash being sponsored by Adobe or android being sponsored by Android. However, I just noticed that the performance tag is sponsored by Red Gate.
Does it really make sense to let someone sponsor a tag that represents a general concept? I mean, if they're willing to pay for it, so be it. But it just seems silly to think that someone might be able to sponsor tags like file or memory.