As per participate in Meta privileges you do not require 5 reputation on Stack Overflow to participate on Meta Stack Overflow. I know the reason because Meta Stack Overflow is also Meta Stack Exchange. So users who don't have Stack Overflow account can also post here (e.g. discussion related to Stack Exchange).
But my concern is that why do we allow to post on Meta Stack Overflow who don't have any associate account(at least one)? For e.g. this user* was able to ask an off-topic programming question without having any associate account.
IMHO to make a meaningful post on Meta, you should have a Stack Exchange account.
Is there any reason behind it?
* He just associated after asking that question