Military contractors and security companies use military clothing for many of the same reasons we do: to differentiate them from non employees and to take advantage of the many pockets, attachment points and body armour that modern combat clothing has. Wearing the same uniforms as an existing army in theater has some strengths (the locals will equate the uniform with the army, and may expect the same capabilities as well. Also this makes intelligence somewhat more difficult for the Jihadis, since the head count will be quite a bit higher), and one very obvious weakness (western soldiers are targets by default, so wearing the uniform signals you are a target).
Now companies might be able to get the advantages of point one by contracting their own uniforms (somewhat expensive), or going to the surplus bin and buying up old CF OD Green combats or East German uniforms, but that still self identifies employees to the Jihadis, most of whom don't care about the distinctions between soldiers and contract employees anyway. As for the line between combatant and non combatant, the Jihadis don't make that distinction anyway; if you are a Westerner, you are a target.