Why do they always want something permanent?

So far, I've worked with two different sites in creating forum skins. I don't know if it's my method in working, or just having weird clients, but they always expect me to keep working on their site, updates, and all that stuff till....the day I die it seems. I just worked with one guy, created my first ever actual contract, then he said to me "I'll also be needing you to alter graphics on the site for me." Is there some kind of flaw in my approach? The steps I took were

- Talk to client on phone.
- Go over everything that the guy wants from his website.
- Write contract, with proposal outlining everything I would be doing, adding how much extra work would cost (also including what the extra work would include.
- Giving it to client.

Every client seems to presume that I'm there with them for the long haul. Am I missing something here?