Me, myself and I along with many others, hate nothing more than looking at a mod and thinking its interesting, then clicking on it to find its crammed full of content from real games or other legitimate mods, but although you look around the page, you find no words saying they actually got written permission from the original makers to use the content, at this point i class them as illegal mods (piracy mods).
If you sit there and think of an idea, then say "I know, i'll make a modern warfare mod", but then you say "my skills in 3d modelling are non-existent, and would take along time to make them myself, i know i'll take content from a modern warfare game i have here". You will then possibly look at ARMA 2, find tools to get your hands on the models from the game and then proceed to convert them to the game you want to make your mod for.
During this you may have thought/or even not thought about asking the developers if you could use this content, possibly thinking it will take too long and want to get started, or possibly afraid of being refused permission, so there you are making this mod with stolen content, which automatically classes your mod as illegal.
I've seen many people say "The worst is the mod will get closed down, but they will just make another and another", but have you ever thought why the mods get closed down, this is to protect you, as the company could come after you with the legal bat.
If the ARMA 2 devs came strolling around looking at mods coming out, then see yours with their content in, but they have no record of you getting permission, theres many options they can go with:
(1)-the lesser is asking you if its for personnal use or whether you are intending to ask for permission to use that content, which can lead to the next one
(2)-the most common one is they will contact the site and get the mod and you removed from it, then this can be taken a step further
(3)-the greater one which they protect you from is the legal side, many people don't even realize this one, when the company sees a little benefit, this is where you can really soil your pants in fear, as this way they can sue you and possibly get some monetary value from you, possibly even get you imprisonment, this seems a more likely option with more companies and governments getting fed up with piracy.
Many can get away with it and release their mods, then other people get the content from that mod and before you know it, theres a chain of them using the same stolen content. You may think your in the clear, but you'll be leaving a trail and can come to get you far into the future.
So the end of the story is, if your going to make mods, atleast make the content yourself or gain it the right way, thats either getting permission or whether you can use it, then you'll know the legal bat is never going to whack you across the head. For me i can make a model in a day if needed and have it funtioning, i even go and ask for permission to use stuff, so why can't YOU! do the same?