Timeline and Newsfeed are two very different things. The feature in question is the newsfeed.
If you go to somebody's timeline (previously "wall") you will see every post they've made in roughly chronological order.
Is that reason because things are roughly sorted by time?
It's a timeline of interesting events in the recent history of your friends and interests. Being a "timeline" doesn't mean that it has to include everything. Should a timeline of the history of the USA include every event in the history of China? Not if you actually want to succeed in communicating any subset of the information on the timeline effectively...
People expect to sign in and see stuff about their friends. They don't expect to have to hand manage every single connection they ever make to decide who is or isn't worth paying attention to.
> Twitter works that way and nobody thinks it's broken.
1) Twitter has extremely different use cases.
2) I think Twitter is horribly broken. I rely on non-broken external clients to sort and filter tweets; most high volume users do.
> If someone spams your timeline it's really easy to unfollow and solve the problem.
Again, Twitter is a totally different use case. I might not want to unfriend my Aunt, who loves me very much, but I sure as hell don't want to see her every little post. I don't interact with her messages, so Facebook stops showing them to me. That problem seems solved to me...
People whose FB posts annoy me, I can easily remove from my newsfeed without unfriending them or waiting for the algorithm to kick in.
Really, though, it's good that they have this. It'd be much better if this and a friends-only feed were the main way they distributed content.
The internal heuristics generally seem to get it right eventually: I see true friend and family posts with essentially random delays of sometimes up to 2-3 days, but I don't actually miss any (I don't think).
I don't even bother trying to follow stuff I'm interested in on Facebook anymore. G+ does that sort of thing better anyway, at least for all of my interests.
If you think that you can complain "But they are my fans! They belong to me! I deserve the right to be in their feeds" then be happy that Facebook is smarter than you: All those lovely fans would be gone in a heartbeat if Facebook didn't protect the advertisers from themselves. If you and all your advertising friends had it your way, you'd spam users until there were no users left to spam.