The basic mindset behind those investors unwilling to buy from other domainers, is not liking the possibility of being on the receiving end of their own game i.e. being flipped to, at a higher price than the original investment.
But for expiring auctions, the perception might be different, since no one in particular(domain investor) is auctioning these domains off and these are considered prospective/valuable investments falling out of the sky, which if you don't grab, someone else will.
Of course in the above scenario the registrars are waived off as neutral parties, even though they directly profit from these expired auctions.
While there is nothing principled about not buying from domainers, an expert investor is one who can look beyond any bias or prejudice, and sees every domain for what it is, through the lens of its investment potential.