You can look at this from a different point of view.
When I was a sprog travel agent back in the very early 1970s I was asked to book a group from Geneva to London. I got onto BEA(!) who said they couldn't do it.
The person who had asked then travelled to Geneva and visited BEA there. They sold the required seats in the form of little groups of 4 (which was the maximum they could sell in one transaction - 4 at a time). They didn't ask "groups" in London. Effectively they overrode the system, completely messed up any kind of yield control that BEA ran in those days and quite possibly oversold the plane.
If airlines are going to make money they need control over the sale of their product. And that can mean that we sometimes can't get the seats we want.
Now think about this. BA operate an early flight London Paris every day. Most weekdays that plane will fill with business people who book late. Therefore, if you try and book a cheap seat on that plane you probably can't; they'll only sell seats that command a higher fare. And it gets worse (or better) because not all seats (at the same fare) are equal. The decision whether to sell that seat can also depend on who is asking. They may want to retain seats for people connecting from their own (or other oneWorld partners) flights. Air Canada may be doing something along those lines.