In the US, It's legal if you have a license. That's covered by part-97 of FCC regs (highly recommended reading for insomniacs).
Transmitting RF *might* be legal if you don't. It all revolves around it being a part-15 compliant device (like your cordless phone, wifi, or wireless security system).
For part 15 compliance, the OEM must have their device measured, certified, and registered with the FCC prior to sale -- many of the "new gadget in the works" leaks are from public records of this testing. Along with this are certain design considerations, like making the antenna non-removable/non-modifiable/non-standard. This is so you can't go in and "boost" the power by swapping that whip antenna with a "better" one you bought at radioshack (as if they'd have something better
). There are also signal type considerations -- digital like our control radios is fairly loose, power wise, where analog like an FPV video signal is typically allowed only a 10th of the power.
Again for you to use it legally without the license, the OEM will have needed to go through this design and testing process. Unfortunately, if you want an FPV system with any reasonable range with analog video, the system will have to have higher power than allowed by part-15 and therefore for it to be any good it will completely fail the certification process.
Think of your control TX -- it's got good range, but it can tolerate a high error rate before it drops control. Now cut the power by a factor of 10, and demand a good signal to be useable. Control Line FPV anyone?
There are some digital video systems that are trying to take advantage for the higher radiated power. Promising, but many of them are still very short range because the bandwidth is much larger than the control signal and the error rate MUST be much lower.