Most of the missions I've designed in the past and with FC and both DCS BS and A-10 have SHORAD, mostly in the form of SA-13 and ZSU 23-4, present as has been pointed out in this thread it is A: realistic to expect to come up against them as they are small fish and passive systems until you're in close and they engage, and B: it makes things more interesting and challenging!
I have also placed units like the SA-10 and SA-11 in areas to prevent players from wandering off, but in a lot of circumstances, even when I write it up in the briefing "do not go here" people see the spike in the TEWS and decide that the target needs to be engaged regardless :(
Not all mission designers are Dumb, or Evil for that matter, we're just trying to create some immersion!