Those who are getting the email in the first place, in a large proportion of the cases, are getting it because they or someone they have transacted with have displayed behaviour that make it clear they are not engaging in legitimate family transfers (perhaps by having a seemingly large family comprising tens of transfer partners who are also transacting with many other members, transacting frequently to a large number of accounts and ultimately perhaps admitting to it), at which point I'm not sure the definition of the term matters so much anymore.
If everyone acted carefully, rationally and transacted sparingly, you'd never need to explain yourself. But you'd be relying on complete strangers (if you did buy/sell points) to also act rationally and this is the point at which it all starts to fall apart.