Many times it's done without the author's permission, and without giving credit. If you see an uncredited model, don't download it and email the webmaster telling him why. Mod authors in general do not want their models converted for use in another mod, so please don't ask me to do conversions, and do not send them in for review. This includes models from Gearbox and Counterstrike. They have explicitly asked that they not be converted.
Most people ignore team colors when they create new models, and thus reskins are usually the same.
For some, it's difficult to plan. You can read the skin's readme file or simply test the skin out in the game. Better not to do it in a real game though. Some of the skins posted are team-color capable while others are not. You can determine this on the left side of the review.