When is a bug not a bug?

I am sure the Tester at the place I currently work at secretly hates me. He is far too nice and far too professional to ever say... but i am sure of it.
He is in a position where he knows what the application we are building should do and how it should do it. Unfortunately this is only ever communicated to me after I have built the module. He doesnt write the use case so dosen't get a chance to prewarn me.
These changes are raised as bugs (as we only have the concept of bug, no concept of a change request) and so it looks like my code sucks as i have a couple of dozen bugs logged against my name (which is standard across the team). I dont like the idea of my code sucking. While looking through the buglist today, after reading the first 12 and realising that they were either A) Not bugs, but changes or B) not my bugs to fix; I got a little stroppy.
*begin blowing trumpet*
Now, when I get a use case I assume that this is what is required... I know, what an idiot... so I write my unit tests and do the whole Red, Green, Refactor like a good TDD agile boy,
*end blowing trumpet*
however I forget that we only call ourselves agile we are in fact... dreamers.
Huge numbers of tests fail and are left unattended, iteration after iteration... our scope is non existent.. our use case are guesses at what we kinda, maybe want and are completely ok to change at any time with the expectation to deliver on time still intact.
So I have tried to push back.
-A bug is raised
> If in use case ==> FIX NOW!
>Else ==>can be done in next iteration and is logged as "Not in spec/Functional change".

I thought this may rustle some feathers and hopefully means the uses case would be a little more robust. It also means the actual bugs got higher priority, as I think they should.
No...
Now we just do non-functional iterations where we do "bug fixes" on all the functionality that was never originally asked for.
As there are a few developers (6-12) all on UK rates (not exactly cheap) and one BA (still only UK rates, but only 1), I would think it would make sense to focus the effort on the up front work, hell maybe even hire another "BA" so the development team don’t have to handle code 2,3,4 + times.
It also means the teste (again on UK rates) has to test and then retest every time the change is made... how do you spell D.R.Y???
To say this annoys me is somewhat of an understatement. It is pretty hard to be focused and passionate about what you are doing, knowing full well in only a few hours/days/weeks it will all get thrown out because someone threw together a Use case, with out putting more than 5 minutes thought into it.
More time scoping => less time "bug fixing"*

*i.e. retrofitting missing functionality

Now I know Agile encompasses the ability to "handle change", but never getting the original scope correct... ever, through laziness, is not Agile, it's just software cowboy bollocks.

Unfortunately nothing is going to change. This could be a really good project, even a flag ship project for the company as it is using new exciting tools for the comapny (.Net 3.0, Nhibernate etc) and it would honestly only require the smallest extra bit of effort. But it wont and that’s a shame.

end of yet another rant...