Sounds more like people are just stupid. Thailand is the source of about 3% of ddos attacks, the US about 7%. All US users should be hit with the captcha, genius.
genius-meme.jpg again
Don't get mad at me man... Let's look at the numbers.
The USA is almost 5 times the populate of Thailand, and a little more than 19x the square feet land area. So for Thailand to account for 3% of attacks, and the USA to account for 7% of attacks (according to your numbers), mathematically, Thailand has a more concentration of attackers (whether you look at it from a 5x the populate standpoint, or the 19x land area). In order to be on par with the USA's DDOS, Thailand would have to be at 1.4% DDOS attacks to USA's 7% (populate route). Or 0.37% to the USA's 7.% (land area route).
So yeah, you've got more than double the amount of DDOS attackers than the USA (populate), even though the USA occupies 19 times more land area than your country. That's what you called concentrated, so that's the reason.
Again, Good luck bro.