Sadly not everyone does that. I know (personally tried) at least one internet online movie service (you pay and get to watch a movie online) that has built the service so that it only work with Explorer and ActiveX plugins, meaning that it's very difficult to make it run under any other OS than Windows (I didn't try vmware or such things, for obvious reasons..) -- after sending an email and waiting for a few days there came a short answer that the service was built on "Windows-only technic" and that it was going to stay so, because it was "the easiest approach". Not sure what they meant by that, but it cuts quite a lot of potential customers off them.
It's sad that things running over the internet, such as videos, rely so much on platform-dependent blocks. After all, I don't think it (internet) was ever meant to be platform-dependent, probably just the other way around (or why else are websites written not in Visual Basic but html, php, ..?)