They are legally bound to contribute their modifications back to Linux (if they distribute them), it may be a different matter when they don't have to.
I can't say I know either way, however I do recall their Tizen SDK licensing not being accepted as open source due to it only granting the right to use certain components on 'Tizen Certified Platforms'.
No.. they are obliged to provide the source... not contribute them back. (Although the cost of maintaining patches internally, makes it cheaper to contribute them back)
