From the United States Copyright Act in 17 U.S.C. § 101:
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a “derivative work”.
from wikipedia:
"For copyright protection to attach to a later, allegedly derivative work, it must display some originality of its own. It cannot be a rote, uncreative variation on the earlier, underlying work. The latter work must contain sufficient new expression, over and above that embodied in the earlier work for the latter work to satisfy copyright law’s requirement of originality."
Because they make tribute items that are creative and vastly different from the things they are parodying, they're not going to get into trouble.
The only thing I see which is too close is maybe the ninja headbands, but I'm not an intellectual property lawyer so I couldn't say for 100%.
The rest of it falls under the derivative work protections.
Otherwise every parody ever would get sued.
It's also why those homestuck dicks had no grounds for any kind of legal anything.
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