There are different issues being mixed up here: the fairness of the policy, the enforcibility of the policy and the motivation of the policy. I am only speaking to the motivation of the policy. Obviously we are just trying to read minds here since neither of us has any evidence of what went into their decision-making process. I am simply telling you from my first-hand knowledge in this industry, that 1) infiltration by potential competitors is common 2) it is a real concern 3) a non-compete provides some mitigation in terms discouraging the practice and providing a possible avenue of enforcement (though it might not actually be enforceable)

The blanket way OP has been dismissed really has to do with the rustled jimmies about the fairness of the policy and does not reflect well on the quality of the replies.