The client might cluelessly think that they're doing you a favor to arrange your travel. Have you tried offering to do it since it's faster and more convenient for you to just do it yourself or perhaps even saying that (if true) you have an eager assistant who enjoys doing the travel arrangements for you and is extremely good at it? Certainly it should be obvious that from your end, it's easier if all travel is arranged in the same way, on the same calendar, etc., if only to avoid potential conflicts and other mistakes.

It may be too late, but control of your own travel (and whatever guidelines, or just include it in the total price) should be part of your contract with the client.