Most of the time, I'm using back-to-back/nested/cross ticketing to reduce travel costs.

Some years ago, a representative from KLM called me to discuss this 'as it was not allowed'. I politely asked her if I understood her correctly that she wanted me to book every second ticket needed to let this work at another carrier? Was she really asking me if I could please take 50% of my business elsewhere? She asked some time to talk to her supervisor. I hang up and still awaiting her feedback.