В чем разница между комплектом разработчика и оборудованием с открытым исходным кодом?

Companies like Texas Instruments either charge for their development kits, require you to develop on special developer's hardware, or require you to license your final product for use on their platform. To get your hands on the specs, they may demand you to sign a nondisclosure agreement, they may enforce restrictions on how their product must be integrated with yours, or require you to give up certain rights to your works. Even where they do not charge you for the specifications or licensing, they still retain the right to sue you if you steal their designs.

Bottom line; they own the hardware AND the core software that runs their design, and you are only allowed to use it the way they say you can use it.

As for open platforms like Arduino and Raspberry Pi, the license to use, copy, develop, or distribute products for their platform are freely available, and they maintain no rights to your derivative works. This applies equally for both hardware and software. You can use the designs any way you choose or even make new designs based on theirs. Or you may integrate their design into a larger product if you wanted to. This is one of the reasons why these devices are so popular.

So yes, you could just start making copies of Arduino's board and sell it as your own if you wanted to. But you certainly could not do that with a TI graphing calculator.

Open source hardware must come under certain licences in order to be considered open source. These licences tend to ensure that the user is able to do whatever he or she wants with the hardware and that the developer must provide the schematics to said hardware. This means that the user is under no obligation to the developer and that the developer relinquishes all control over product. This not the case with most development kits. If the development kit is indeed not open source (which most aren't) it may come under any licence and said licence may require you to do with the hardware as the developer dictates. This may not be much, but generally these licences require you to observe strict rules and royalty obligations when releasing the product you have developed with the hardware in question. These licences could in fact contain virtually anything and they don't have to be approved by any kind of established authority. This is unlike open source licences which generally have to be approved by organizations such as the OSI or the FSF before they can be considered open source.