As some of you have deduced, the reason we added the extra clause was because we were concerned about supporting modified clients on our rendezvous server. We want to provide a good user experience for everyone using Tandem, and it would be difficult to do that by supporting modified clients as well. Like you pointed out, we don't want potentially bad forks to cause problems for others. The spirit of the license was not to stifle the abilities of others to contribute to or distribute Tandem.
With that said, we’ve heard your feedback and agree that this kind of stipulation is better suited for a terms of service on the server we host. So we’ll be switching to the unmodified Apache 2.0 license and adding a terms of service to request users of modified versions of Tandem to use their own hosted server.