...how come youtube.com has rights to steam movies online for free?
They don't, always.
When you see full length movies on Youtube
et al, it's almost never with the consent of the entity that owns the movie. That is, it's an infringing copy - a "bootleg".
Often the bootleg has been unlawfully copied ("pirated") from a legitimate copy
(such as a store bought DVD, or a "screener" - a copy supplied to a retailer to play in-store).
At other times, it's a hand-made infringing copy - such as iPhone footage shot of a cinema screen.
There's so much of it that the housekeepers at Youtube (etc) basically can't keep up.
Sometimes though, the copyright holders (say, a movie studio) will supply ("upload")
content that they agree can be shown on sites like Youtube.
This usually is to promote other content (or other formats of the same content).
Movie trailers and music videos are examples.
Sometimes, people (like, say, record labels) upload content, and then (by agreement with Youtube)
take a cut of the revenue from advertising that Youtube places on the page. Vevo, for example.
Korean pop artist Psy's connections made a packet doing exactly this.
Understand this. In Australia, if you buy a legit CD and rip it to your iPod,
or a legit copy of a movie and rip it to your laptop, then no problem.
You often hear this called "format shifting".
But if you make a copy for someone else, that's infringing.
What you propose to do is to infringe in an organised, large scale way.
Don't.