Conceptually this isn't all that different from having a website that works in browser A but not B due to insufficient testing. Why did they not fix it?
Because how many website operators continuously google their own web site on mobile devices to click through to experience their google-hosted AMP editions? I would wager it's a fairly small % of the number of websites running an auto-AMP-ifying wordpress plugin, for example.
I've seen plenty of badly designed mobile websites over the years and these sites seems to have turned out okay - most mobile browsers keep the option of "Request Desktop site" very accessible for a good reason.
The worse case I've seen is a site in which every page crashed Mobile Safari without fail regardless of which version you ask for. It was eventually fixed but I never figured out why. If the sites are just running some script without checking then the admins have failed their line of duty.