They don't normally delete posts.
They do ask you though to submit proof that you are/where a customer, so that they can ensure that it is a legit review (as in you are actually a customer of theirs).
Personally, I do not see anything wrong in this approach, it weeds out shills, or people posting bad things about one of their competitors. Like anything though, it is not going to be perfect, they will make mistakes.
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