You think other retailers don't have the same issues?

No they do not. For example, every single memory card I got from amazon was fake, even if I explicitly selected "Amazon" as seller. Meanwhile every single card I ordered from Newegg was original and high speed.

How many memory cards did you order that you can measure both? I know Amazon has a problem with counterfeits, but it's a bit hyperbolic to say all Amazon bad, all Newegg good. Anecdotally all the controllers I've bought from Game in the UK have been sold as "new" but actually open box and used, whereas every one I've bought from Amazon has been legit.

Is that a problem specific to the US?

I have ordered a few memory cards from Amazon by now in Europe, and I've never had issues with them. I don't think I have ever received a fake item from Amazon. Not where I've noticed it anyway.

Disclaimer: Work at Amazon Not sure? I built a new PC recently and didn't run into any problems with counterfeits when I got everything from there

Weird... I have ordered a dozen or so memory cards from Amazon and have never received a fake one.

How do you identity a memory card as fake? Did it function?

You use CrystalDiskMark to benchmark it. Visual inspection as well. Most real cards are made in China, knockoffs are made in Malaysia but its not always the case. Counterfeit will be 1/3rd of the speed of regular one and most likely much shorter shelf life since its pretty much old generation chips used.

I build custom video setups as hobby so I say overall probably I got over 150 cards from Amazon (at least half were fake) and some 100 from Newegg so far (last 3 years) and haven't had a single fake from Newegg. [mostly I use Sandisk and Lennar cards]

For many smaller items, I prefer the ease and convenience over price differences or occasional bad products. I know I'm not alone in this and it shows how much experience matters in purchasing.