Of course unlocking weapons with third party programs IS cheating.
The thing is, however - from another player's perspective it is COMPLETELY irrelevant how you got your weapons. Everybody has access to unlocks sooner or later, you don't cheat during actual gameplay and you didn't spoil the fun for anybody while unlocking your weapons the other way.
So yes, I cheated - just like I cheated when I typed in "greedisgood" when playing Warcraft III campaigns if I found a certain mission too frustrating. So what? The Steam Achievement Manager doesn't even work inside a particular game, it's purely centered around achievements thta in every other game don't give you anything and in TF2 serve as a cockblock to delay getting your hands on something. I wonder how some pro player would feel if he/she had a match the very day the unlocks appeared...
As for the achievements themsleves, I've told you already that many people enjoy them and like the system. It's the reason why it's present in so many games. I'm trying to make you like them, that would be silly of me, I'm just trying to tell you that not enjoying it yourself does not justify cheating. That's all.
Of course not - BUT the fact that you oppose achievements coupled with the fact that you do not harm anybody or the gameplay itself and from another player's perspective it is completely irrelevant sure does justify that in my book. I cheat the system, not other players and hence I have absolutely no remorse about doing something wrong.
If you fire up an achievement map with everything in place and let some friends join it takes around 5 minutes per player to unlock all the weapons for him - so what's the point of even attempting that? The weapons are rewards for what exactly, if they are so easy to get? If it's only a question of time before I get something, then I choose the shortcut. That's also the reason why I don't play classic MMORPGs anymore - time became an important commodity for me.
P.S. I wonder why you became so righteous, Bart - you got all the unlocks (not all achievements, but still) in previous class packs on achievement maps - I know, because I was there. So, how exactly does that differ from using the Steam Achievement Manager (not from a legal point of view, since you're breaking the license agreement, of course) except for the fact that it takes a couple of minutes longer?