Sites like IMDB as well as Rotten Tomatoes and countless other websites or mobile applications (such as flixter, etc.) all use movie posters and images from movies as part of their content which is more or less the selling point of their business (or maybe more accurately how they convince users to keep using). How do companies get away with using images or movie posters for free, does that mean movie posters and content are fair use for commerical use?
It would be an overgeneralization to conclude that movie posters are "fair use for commerical use".
The uses you see imply that low resolution reproductions of movie posters have been judged by IMDB, Wikipedia, and others to be fair use for the purpose of identification alongside commentary (or in a transformative function like browsing).
As an example case, see Bill Graham Archives v. Dorling Kindersley Ltd., 448 F.3d 605 (2d Cir. 2006), which held that it was fair use to reproduce thumbnail sized Grateful Dead concert posters in a book alongside a timeline.
For wikipedia: They will generally use a scaled down version under fair use as nomen agentis explained. If you look at the meta data for the images, it states how it is fair use (See Inside Out Poster). If it is found that the uploader did violate copyright by using a copyrighted version of the image that is not covered under fair use, then a DMCA notice is issued and the image is removed or replaced.
For IMDB: IMDB actually pays for the licence to show the photos. Per their FAQ about using images from their site, they state that the images are licensed to them only.
IMDB also charges a fee for high-resolution photos of the posters. This is an advertising fee more than anything else.
For other sites: they may do the same as IMDB unless it's a community site