This is a bug. A bug in design, perhaps, but still a bug. A limitation, too, I imagine, that no one wants to step up and solve (myself included). But still a bug.

It's a limitation imposed by the design of HTML forms, which requires significant web-author effort to work around. It results in some very non-intuitive behaviour, and thus it's at the very least a bug in the design.

Is it worth it to fix? Probably not - almost every page you look at has multiple forms, and the state of other forms are not kept when submitting other forms. That's a lot to go through. But just because there has been, and continues to be, a conscious decision not to change the behaviour doesn't mean it's not a bug.

Sorry about the rant - but I have to deal with this line of thought at work, too, and it annoys me to no end. I have no problems with shipping bugs. As long as everyone acknowledges that's what we're doing. Call it a limitation - fine. But it's still a bug. A bug we may not want to pay to fix. But it's still a bug.