This used to happen to be at least a few times a week when I lived in Chicago and worked in NBC tower. I would get stopped and talked to harshly before I whipped out the magna-ID and got the apologetic, sort-of embarassed version of the same guard. The motivation behind it really depends on the person. I used to think it was all about power dynamics, but in a lot of cases these guys are really just doing their jobs. They didn't make the rules. I look like a messenger. Hence, these guys think I'm a messenger and want me to follow the rules that have been set up by the building's landlord for messengers. It's all about how the guards approach you. If they're rude and gruff and disrepectful, then f'im.
But remember, sometimes everyone has a bad day. And being a security guard kind of blows. Still, they should be courteous, because that it part of the job too. I don't know. It's a crappy situation all around. Crappy situation for the messengers, crappy situation for the guards, and crappy situation for me because I have to go to work in stupid office building and sit in a cube all day.

Viva la revolucion.

J