From a service provider’s perspective, a professional who is on travel isn’t fully available for other work. So (says the provider), the customer should pay for that opportunity cost.
From the customer’s perspective, the opportunity cost a professional incurs in traveling on business is something the provider should absorb in its internal cost structure. This is especially true (says the customer) if the traveler might be doing work for other customers during the travel.
A possible compromise might be for the customer to pay for travel time at a reduced rate.