Yes - depending on the movies and facts not known here - some streaming from stagevu.com may be copyright infringement. There may be other issues as well, such as unfair business practices.
Copyright is a type of intellectual PROPERTY. If you download (or "just stream") a movie without the copyright owner's consent, you may be directly infringing the owner's copyright. The owner of the movie is the only one with the right to reproduce the movie, distribute copies, display the movie publicly, etc. If you and others are watching it without the owner's permission, you are stealing the owner's property. This is not unlike the unauthorized music download cases that have been in the news for the past few years.
Turning back to your question, it suggests that you aren't "downloading" the movies to your hard drive or burn it to a disk. However, even when you are "just streaming," your computer may be "caching" some of the movie content for brief seconds. For instance, If "Fast Cache" is enabled on a Windows Media server, content might be cached to enhance playback. Therefore, you may be reproducing portions of the movie during streaming whether you know it or not. And you probably are displaying, "performing" or distributing the movie without permission. See 17 U.S.C 106. So, unauthorized streaming, like unauthorized downloading, can be direct copyright infringement.
You should also note that just because you find something on Stagevu (or elsewhere on the internet for that matter), it does not mean the owner of the copyright in the movie authorized Stagevu to upload and offer it, or that you are clear to use the movie downstream. In fact, Stagevu.com's own disclaimer admits there may be issues with some of its material that you might be streaming: "Stagevu respects the rights of copyright-holders. If you are a copyright owner and feel that an uploaded video infringes your rights, please fill out this form to bring it to our attention."
Streaming or downloading of music, movies and the like is not a harmless bad act. It deprives the creators and owners of their property and livelihood. It's easier and nicer just to pay Netflix or RedBox $1 and rent the movie.
"UNAUTHORIZED streaming or downloading of music, movies and the like is not a harmless bad act."
And one further point relating to this topical question about streaming:
Copyright law includes a concept called "Fair use." Fair use permits "time-shifting" and "space-shifting." Time-shifting is, for example, recording a TV show in order to watch it later. Space-shifting is like ripping music from a disc to computer, then saving it to an iPod for later listening.
So, AUTHORIZED internet streams can be time shifted and space shifted for personal use, just like saving a TV or radio show, but copyright laws still prohibit distributing copies of the saved stream, publicly displaying them, etc.
And if the stream is UNAUTHORIZED to begin with, it would still be UNAUTHORIZED when the downstream user streams it. Illegal streams can't be washed clean.