While I'm not in favor of IKEA's policy and neither do I have anything against the website, the IKEA position seems to me to be legally correct.
The site uses the trademarked name within its own. This is not factual use in text, it's nomenclature and logo. This is exactly what trademarks are supposed to protect. It might be different if the site name was different, and if it merely described itself in text as being a site for people who want to hack IKEA furniture. Then it could just note that IKEA is a trademark of IKEA in the text. But this website uses the protected name directly in its own, with the intention of associating itself with the parent brand.
IKEA's lawyers have no doubt advised management that if they ignore any trademark violation whatsoever, they run the risk of losing their trademark entirely as other commercial interests can then argue that IKEA has chosen not to protect the mark and has allowed it to become generic or public.
Of couse the reasonable, intelligent thing would be for IKEA to specifically permit the site to use their name, since they are only providing IKEA with good publicity and higher sales anyway. That way they aren't ignoring the "violation" they are authorizing it, their trademark is safe, they don't piss anyone off, and everyone is happy.