Exactly right, due to "security concerns" you were not supposed to run anything other than IE6. In this case, "security concerns" are defined as: "the club by which a corporate IT department can beat down a reasonable request without being forced to have an intelligent discussion". All machines had monitoring software that phoned home regularly to tell IT what software you were running.
Most average employees just need to get their work done, so they are not going to risk a fight with the IT machine over a browser. To non-geeks, it's just an annoyance, not a major blocker. Better for them to save their energy for a fight that matters (like getting an exception to the 100 megabyte email quota).