Hey Rico mate!...lighten up!
Get your priorities right and into persepctive.
You're modding for a game, it's a hobbie and for some people looking for a career in modeling etc it's practice!
It's not like anyone on these forums is making any money from this, no companies are in competion for the work here.
Unlike VBS2 there are no liscenses for ArmA that can be purchased to allow you to work as an independant contractor!
If someone wants to start a mod and develop ADF assets "their" way then what's the big deal?
If they happen to have re-textured a bis model that looks similar to yours then so what!
If people in the community like someone else's version of an addon better then the one that you've made then so be it!
And if you give permission for someone to use something of yours then you have given permission...period! You have the choice to say no from the outset.
Asking permission to use someone's else's work is the right thing to do, like SoldierX said they would have put a lot of hard work into it. There's a certain amount of IP that comes into play, but again you can't sell this stuff.
But you can use it as part of a show reel if you're looking to get actual paying work in this field, so being able to show it is actually your work is important (or more importantly being able to catch someone passing your work off as their own).
But if you're not practicing or building up assets for a showreel then it's a hobbie, something that either drags you away from RL or helps you escape it...that means it's supposed to be enjoyable and not stress you out, and if it is maybe you need a new hobbie.
If any of the points i've made are incorrect i apologise in advance, but the point is easy up mate, there's a whole wide world out there beyond your PC and the internet that's much more important.