Because they want to have to do with it
Okay, I assume you aren't saying that Rabbi Bleier has negius because he wants to make money on the hechsher, but rather, are just joking around. I've heard such an accusation leveled at Rabbi Berkovits, though I happen to know that Toras Habayis (his hechsher, founded so that people can actually afford some hechsher leshitaso) makes no money. But you obviously don't mean that.
“The poskim extend their validity” makes little sense as they don’t have the authority to do so.
I don't see what the problem is with extending validity - is it any worse than making a new takana? Who decides what the poskim have authority to do?
See footnote 28
Very nice. Happy to see someone in writing saying that one has the right (yitachen, at least) to trust someone who isn't known to the rabim.
Regarding what he says before that - which is that someone can buy from a mukar larabim even without a hechsher - is something Rabbi Berkovits says also. His problem is that you can't really have a business where someone random calls in, and you tell him, "Sorry, you don't know me so I can't sell to you."