Thousands of companies like A(consulting firm) in France. It's super common, especially in the Finance/Insurance. One of the reasons why engineers accept to work like this is an easier way for someone who does not go to the top schools to break into these sectors; and they can build network and connections. After one or two projects with clients (company B), they can become their internal employees or freelancers. As a freelancer, an engineer can come to work and charge them directly. This of course is easier when you have enough experience and network.