I've seen this fairly frequently in the past, AA in particular were very aggressive about blocking this type of tool. Technically it's in violation of the TOS for the same reason that the automated upgrade inventory tool mentioned here a few weeks ago was blocked - the bit about using automated technology to pull content from the site:
User agrees not to use any robot, spider, other automatic device, or manual process to monitor or copy this Web site or the content contained therein or for any other unauthorized purpose without United Airlines' prior written permission.
http://www.united.com/web/en-US/content/legal.aspx
There are probably two reasons they do this. One is the incremental load that this could put on the site if lots of people were using this type of tool and each of them made frequent pulls of the data, the other is that it creates large repositories of united.com account login details on third party servers where they have no control over encryption, etc. and this probably makes plenty of people in United IT uncomfortable.