Based on ExpertFlyer's response you posted, there's nothing there that would stop me from being a customer of theirs, if that's the only way for me to get the availability information. And I don't know if KVS's policy was any different. If a KVS subscriber wrote some software and allowed a lot of people to use it for free which just ferried requests to KVS and returned the results, I imagine KVS would likely revoke their account, even though it seems that's really all KVS does to begin with, gathering information for free from other sources. Guess I'll need to ask ExpertFlyer how they gather their information, if they're willing to share. Though ultimately how they come about their information, provided it's done legally, doesn't bother me. Nor does it bother me if I sign up for a service with the agreement that it's for a sole individual's use, as long as it's spelled out ahead of time.

Edit: I just went and had a look at the flightstats.com thread and found EF's response there:
So if I'm not mistaken, EF paid to get the information directly and legally, whereas KVS looks to have been getting their information i an indirect manner, and possibly for free. Well, for me my choice will be perfectly clear if I need to decide between which service to use.