I have a few associated accounts. When I go on another Stack Exchange site, I'm tempted to associate it for the 100 point bonus, even if I'm only reading and not posting. Is this greedy? Would it be unethical to associate with sites I don't even read for the 100 point bonus?
You're supposed to associate your accounts. That's to help the system much more than being some sort of reputation-grabbing, free give-away for the greedy.
Remember, reputation is a rough measurement of your experience with the system. The +100-rep association bonus is to give you a quick boost for the experience you have already EARNED on other systems. That bit of reputation empowers you with some of the most basic (and sorely needed) resources needed to be spread across the sites.
There is absolutely nothing unethical or greedy about it.
It's not greedy. It isn't like there's a limited amount of Rep to go around. The whole point of the bonus is so that users who have shown that they have experience with and have proven they can be trusted on one SE site are allowed to have the basic privileges on another.
What difference does it make? You don't get any more points on the sites you're already using (the bonus is one-time per site), just the new one, and having points is irrelevant if you don't use them.
By getting 200 points on site A you've demonstrated you know how to ask and answer questions, vote, comment and otherwise participate in the site.
By giving you a 100 point bonus the system is acknowledging that level of expertise and allowing you to do it from "day one" on site B.
By having more people who can do these things and have demonstrated they know the way Stack Exchange operates site B will be a better place and will (hopefully) be more likely to succeed.
Why should it be unethical. You can add the site, follow it and decide it is not for you. Or you can save it for later.