Consultants: When not to extend?

I am new to consulting so I have not run into all the possible problems that can occur.

One that I have run into recently bothers me but my boss tells me it is normal for consulting.

I had plannned a vacation a while ago and my boss knew it. He then extended the contract at the place I'm consulting at without telling the customer about the vacation.

I think this is unethical but as I say the boss says it is normal.

Any opinions?