Agreed. IMO there are a handful of KILLER Swedish singles (220 Volt, Scratch, Hels, etc) but most are too lightweight for my tastes.
Agreed on nwobhm too- there are lots and LOTS of substandard, boogie, AOR, and other tripe mixed into that category.
US metal has it's fair share of AOR, glam, power-pop, etc mixed in, but there's plenty of tr00 Steel in there too.
So why are US singles criticized more? Good question. My theory is that it's an example of backlash to a new trend. NWOBHM and swedish singles are, as someone said, pretty well established in the minds of fans and collectors. But US singles are the new kid on the block. They're portrayed as 'the next big collecting front', or as DaN once hilariously (and accurately) put it, they're now what you get for the girl who has everything

So we've seen interest- and thus prices- shoot up in the past several years. And what happens as soon as there's a new trend? An anti-trend faction quickly gets established. For everyone ready to anoint FAR EAST Greatest Band Evah without having heard a note, there's someone else ready to write FAR EAST off as some overhyped jerk-off obscurity that must suck simply because lots of people are willing to pay big bucks for it. So one group wants to grab the newest toy released so they can be the cool kid on the block, and the other group claims they're too cool to bother with the latest Flavor of the Month. And in the case of FAR EAST, neither side knows what the record sounds like :lol
Where this leads for US singles in the long-term depends, IMO, on how much exposure they receive. NWOBHM had the Metal Blade compilation and the Japanese boot series raise its visibility. The Swedish stuff had DaN's '707 Inches' to promote them to the uninformed. But currently the US singles are known to such a limited pool of people that they're easy for others to dismiss. If, however, they start to reach the ears of a wider audience, then people will realize that there are a lot of gems that are worth the bother (and the price) and the US singles will take place of pride beside UK and SWE releases.
I think John's book will be the first step, because it will show people how much stuff is out there. The next step will be for folks to get info on how rare each single is and how good each single is. Malc's book provided this kind of info for NWOBHM, and DaN's site provided this for FWOSHM. TMK John's book is not going to provide such info for US singles, and I can understand why- people bitch when you provide prices, and Good vs Bad is VERY subjective. So that info will have to be disseminated in some other form. Bootleg CD series? Dedicated blog? A second book formatted like Malc's? A new Coroo wing called '1,233 centimeters of American Metal 45s'? I dunno. I'll admit I've pondered such projects, but I think John's book must be the first step. Once it is released, then I might seriously contemplate doing one of the above, possibly as a collaboration with at least one other collector-fan of US singles.
Back to the question at hand....
There's also some stigma against singles in general. Many lack picture sleeves, and the price per song is relatively high. And some folks tend to gripe about a single if 1 side is weak, but IMO a single with 1 really good song is quite worthwhile. It's not like most albums are packed with killer songs, so I don't see the problem with a single that's got one good side and 1 not-sogood side. Sure, you'd like both songs to be great, but that's not always gonna be the case.
Sometime I'll have to sit down and compare lists of NWOBHM and US singles and see which side has more 'top' singles IMO. But that's for another time.