Activity is something I too struggle with and I'll admit, sometimes it ISN'T fair.
I've been on both ends of the stick.
The leader of the roleplay wants people who can keep their roleplay going and alive. They want people who will be there, as friends, packmates, clanmates. They want their plots to be carried out and sometimes the RP gets interesting enough to give the character life, and they become attached. They want others to put in the effort that they have.
However, members may not have the same time zone, or even the same schedule of wake-eat-sleep. Maybe they have jobs, school, etc. I wasn't able to roleplay almost all of this year's fall/winter/early spring due to high school and even without me doing any of my hobbies, I still failed to pass the school year. If I were committed to a roleplay, it would be a lot higher stress for me than just that.
But then we go back to how the leader sees it.
It's complicated, and is just but unjust at the same time. I think kicking after 2-3 weeks of inactivity suffices, unless the member has given a notice of absence, though. I've had my share of disappointment when I've been gone for 2-3 days and been kicked. It kinda hurts, almost.