To begin, donations are not in exchange for a particular service or benefit. That is why these are donations and not membership fees. When you donate money to the Red Cross, you are not mandating that they provide fresh water to somebody in Thailand. Instead you are donating the money with the expectation that it is used in a way consistent with mission. So, Carr's claim that they are somehow "making a mockery" of those who donated money "to keep wikipedia live" fails to hold water. This argument is a strawman. They are upholding their mission to provide a knowledge repository by protecting their ability to do that. Again, donations are not membership fees and they are being used at the discretion of the organization.
Second, the neutrality argument is equally weak. This is even addressed in the official Wikipedia release. Providing an objective and balanced knowledge repository does not require that they as an organizations behave in a neutral manner. The Wikimedia Foundation clearly feels the passage of this bill into law would negatively affect their ability to serve the community. Thus, they are expressing their position against the bill. Is that not proper? How personal must the attack be before it is okay for Wikimedia to take a stand and "lose their objectiveness".
His final argument is extremely naive and simply not true. By his account, if you have a stance on a public topic and publicly declare it, you are now and forever inherently stating your approval of any subject you fail to discuss. If you think this is an exaggeration, here is his statement, "The trouble with taking a political stance on one issue is that your silence on every issue becomes a stance...Everything else that’s happened in the world until now, and everything that will ever happen from this day forward..."
By this logic, a group like the Bill and Melinda Gates foundation must be in support of the Rape of Nanjing. PETA must be completely okay with the fraud committed by Enron. I could certainly create an unending list but the point is clear. It is impossible and unnecessary to publicly (or privately for that matter) hold informed positions about every topic.
On a final note, I think it is actually very logical that different services should show support in different manners. Twitter, Facebook, G+, etc. are social networks and as such, can make the most impact by members creating an echo chamber of dissent. Wikipedia on the other hand is a resource. And, just like the physical world, we feel the greatest impact of a resource when it is no longer available.