Not really enough information to go on here. If there was something in the agreement prohibiting "derivative works" it would seem that TD Ameritrade could preve

Not really enough information to go on here. If there was something in the agreement prohibiting "derivative works" it would seem that TD Ameritrade could prevent the developer from making use of the service-derived product, but as you say, this shouldn't invalidate the developer's copyright or give ownership to TDA. The actual EULA text would have been useful here.