First, I really doubt all but a handful of companies would actually do this on account of the FSF site being blocked. It's a simple cost benefit analysis. The cost of replacing such software is high and the occurrences of such mistakes that would actually hurt a company are rare. Therefor it's not happening the vast majority of times.
Secondly, I'm surprised the FSF is using a service that has anything to do with proprietary, closed source, non-free software. Given their philosophy you'd think they'd have found some way to collect donations that uses free software from top to bottom. Maybe I'm way off on this but one of my first thoughts was that maybe even complaining about this takes away just a bit of the FSF's credibility. They preach the gospel of free software but when it's time to fundraise they make an exception? Is this a "do as I say, not as I do" situation now?
I'm not trying to be overly critical and I realize this may be a bit pedantic too. It's not a big deal to me, just thought it interesting. Food for thought maybe.