(Disclaimer: I am not a lawyer. This is not legal advice, just commentary based on my understanding of all this stuff. 'Kay?)
Nominative use of someone else's trademark is legal as long as you follow certain rules. KenzerCo did it with 4E; "Kingdoms of Kalamar" said on the cover that it was "For use with 4th Edition Dungeons and Dragons."
http://www.kenzerco.com/popup_image.php?pID=625&image=0&osCsid=799cb3ad5edbfe55a5410e3f67ff2d23
The reason most publishers don't do it that way is that trademark and copyright law are nasty tricky beasts, full of traps for the unwary non-lawyer. You have to be real careful that your product can't be construed as trying to pass itself off as "official." If you aren't a lawyer and can't afford to retain one, it's easier to play it safe. (Not coincidentally, Dave Kenzer, president of KenzerCo, is an expert in copyright law.)
It looks like this Hass person is not being careful. Calling them "5th Edition Dungeons & Dragons adventures," rather than "adventures compatible with etc. etc.," certainly seems like it could be taken as an attempt to look official. And if he is indeed ripping off the art as well, he's cooked.