Because some people are tricky to work for. For example, the customer asks for A and then gets A, but then has another idea and says "oh, can you JUST do....." ( not understanding what they are asking is extremely time consuming and that the price would have been extra if that was what they had asked for in the first place)
It really does depend on what the terms and conditions are. I don't think its at all reasonable to ask for something to be done and then when it is done, to change your mind or have a new idea. Customers like that are to be avoided. As are those who want something for nothing, or those who have no idea of what they are asking.
I was recently asked to do something that is too physically demanding for me. I would also turn down a freelance job that didn't involve me working in my own environment. I would also avoid working for anyone who didn't seem to know what they might want or who might be looking for a freebie. I have a day job, so can afford to be picky.