Hi Bora,
Yes, you are free to do this. There are no license issues stopping you, and in fact is often a model we've seen people use to do initial trial runs of products/equipment with customers to win contracts etc, where time is the really important thing. Although to be clear our focus is definitely helping you prototype and do trial runs to test your product ideas quickly to get them in front of your boss/customer/VC, not as OEM modules.
You won't have issues getting hold of them; we are constantly building lots of them! The mbed boards are designed as professional prototyping modules that we produce with a long lifetime, so it is not like a one-off batch of marketing evaluation boards. The long lifetime is designed also to build the community, libraries and shared experience around a known platform too, which also helps productivity.
So, certainly nothing is stopping you. Hope it goes well!
Simon
p.s. We have occasionally considered making OEM versions of mbed, with the same form factor but just the target MCU, no programming interface and minimal components on it, so it could be used in production without your whole design needing to be SMT. They'd be a lower cost/zero packaging/bulk purchase affair - if you or anyone is interested in this, please email us at support@mbed.org with approximate volumes and we can gauge interest.
I really love mBed. It helps me so much in prototypes. I know that it makes no sense use mBed in mass-production. But, in some projects that are not in mass-production I would like to use it in the product like a component. Are there any limitations or wrong about it.