A huge part of the problem is the fact that the legitimate promos are few and far between, so to get them people have to jump quickly. And are willing to pay large sums to do so.
These Mexican ones aren't the best done ones. But the recent fakes claiming to be from the Netherlands (White CD in a slimline jewel case, with the ISRC numbers printed on the CD) are pretty good looking and hard to tell that they are fakes. And there's some beautiful fakes out of Korea, that claim to be a EU pressed promo with a nice insert, etc. Those CDs from the Netherlands looked pretty good and commanded a large price, but now they are showing up on ebay and none are getting close to the price as they once did. Word is out that they are fakes, so no one is interested any more. (I think I realized they were suspicious when an "Ordinary Love" promo came out almost a year after the single was promoted. But probably should have been tipped off that the rights society was spelled wrong - an error they later fixed after I pointed it out on another forum!!)