That isn't a way to protect against it, more than it is a way to monetise the site. I'm talking about them having installed an anti-scraping script that'd be an active countermeasure.
Downloading lots of textures in a short time-span. I think, when CGTextures started, they had problems with people running scraper scripts that'd cause problems with their servers, so (Again, I think) they put in some protection against that. It used to be in the FAQ, but I can't seem to find it now.