We use American Express travel for corporate booking. I have seen this for some time now, as
WineCountryUA mentioned, but it is not consistent in my experience. Some flights are restricted and some are not. I have even had the case where I made a change via the web site and when I went to make another change this message came up. You can call and they will override, but I was warned by the agent that if my company has special fares, the UA agents cannot see them/use them. I am not sure how accurate that is.