I used to have a roleplaying group, which is now disbanded because I simply couldn't keep up with it (too stressful), and I'd kick people if they were in-active for a month. Really, it never got off very far, but my point of view on the subject for people that want to know such information is this:
Some people expect more dedication than others, and for that they need to verify that you aren't already too in-depth with another thing, and are willing or at least planning to remain a dedicated and what-not. They expect the best, really; because they recieve the best from others.
Now, my personal opinion on these random kicks without warning or reason is simply that they're irrational. While expecting the best and wanting the best to be online, you still need to understand and cope with the fact that: hey, maybe this person is doing something else? I should give them some time. When people left in the midst of our roleplays, I was annoyed, sure; but I never made a rude comment about it. If they were online on another character and not sitting in the map waiting for me to show up, I was fine with that as well. Cause' people have friends and other responsibilities. While wanting an active roleplay with dedicated members isn't bad, you should still acknowledge these things.
I mean, personally; I think an accepting, not-super-strict-about-activity roleplay that doesn't send you messages about getting online constantly or (maybe even) at all is one of the best. Let people get on if they actually want to, because if they don't have a muse for roleplaying, you're not gonna get good content or feedback from them. If you deem them offline for just too long, give them a warning they know exists before kicking. It's just courteous.
But members should also acknowledge the rules and regulations when it comes to activity, and thus its also part of their responsibility to not get kicked. Sending messages, explaining your situation/yourself and little things like that can really tell the leader something.