Just to be fair I know that NPC is a TechSoup partner but the companies but the feature provider on MerchantSeek doesn't require a personal guarantee or Social Security number either. The processing providers that we work with are very Non-Profit friendly so I can assure you that NPC isn't the only option for Non-Profits not wanting to sign a personal guarantee.
I think ther 3 that you got in touch with are just ultra-conservative as some processors are. But the featured providers that we work with only provide proof that you're a 501c3 and you won't have to sign a personal guarantee or give your Social Security Number, and their Virtual Terminal only option is a tad bit cheaper than NPC's (under $10 to tal per month.)
And to dispell the myth, direct banks are not always the best payment system providers. Often times they're too conservative (as banks can be sometimes) and behind on technology (as banks can be sometimes). A full time merchant provider that registers with a Visa/MC acquiring bank is essentially on the same level as many local banks (they're both registered ISO's) and that full time provider or processor has more knowledge because that's all that they do.
But I'm happy to hear that you're finding luck with NPC (I'll say that they are better than a local bank), but not the only company that offers a real merchant account for Non-Profits and don't require a guarantee.
Just putting that out there. Feel free to ask any questions.
Joe (MerchantSeek)
E: msinfo@merchantseek.com