Parody.

There are entire games based off of other games, and parodying them. "Pyst" was a parody of Myst, and had you walking around environments from Myst which had been vandalized. It was ok in the court's eye.

"Deer Hunter Revenge" had a deer getting revenge for being hunted, a parody of "Deer Hunter".

Unless they distinctly toss in names, like Lightsaber, or Indiana Jones, they aren't crossing the copyright infringement line.

They have a "Very Light Sabre", and "Harrison Jones", and those qualify as parodies.

Of course, there are times when a company does consult with other companies to make sure it was ok.

In TF2, Valve accepts models from players. If the model is good enuf, and it fits a theme they want, they might implement it, and give it stats according to their design, and put it in a future update.

Someone submitted a melee weapon for the Medic called the "Vita-Saw". The design clearly referenced and took inspiration from the ADAM harvester from BioShock.

Valve sought and recieved permission from 2K Games to put the model in the game, and it was released.

Vita-Saw.

http://wiki.teamfortress.com/wiki/File:RED_%C3%9Cberneedle.png

ADAM havester.

http://wiki.teamfortress.com/wiki/File:Bioshock_Needle.png