Isn't this more to do with how TCP and the socket interface is implemented (or more accurately its intended semantics) than a bug in browsers? Not draining any inbound data on a new connection prior to sending my request doesn't sound like a bug.
Yes, that's the practical reason. But a browser could of course have implemented HTTP in the way described in the RFC, instead they all took the easy and obvious way out. That's also why the bug was present in all browsers that had marketshare, it was the obvious thing to do, it is not as if they independently made the same mistake, they independently decided that it wasn't important enough to implement the protocol to the letter.
