So listening to frequency A is okay but frequency B is not?
If you are really concerned and want a final answer, ask your human resources department. Expecting what amounts to strangers on the internet to back up your choice probably isn't going to hold up well.
Ultimately you need to do what is right for you. If you believe you should be allowed to listen to your scanner at work, then go ahead, but remember that they can have a lot more control over what you do that you might want. Sometimes you have to pick your battles.
Keeping things low profile would probably help. If all they see is headphones on and nothing else they won't know exactly what type of radio you are listening to. If you display exactly what you are doing, you can likely expect to get questioned.
Doesn't mean it's right or wrong, just means that you need to find the right person to ask and then decide if you want to follow those rules or not.
Personally I have a scanner going at work, but only to listen to my own systems. I'm in my own office and I keep the volume low.
You are right to question this, just be willing to accept the final answer from your employer, or be willing to look for a new job.