Two excuses IT professionals use.

The following excuses are used frequently by IT professionals as a scape goat for not letting you do something they don't want you to do, or to lay blame on problems they don't know how to solve.

1. Your problems are caused by these other applications that I don't support.
2. I can't allow you to do that, or have that software because it is a security threat.

The first one is used to explain why you are having PC problems. This often comes in the form of some sort of "fanboy" bias. A Microsoft lover might say, "Your computer boots slowly because you have Firefox installed" or, "The Google toolbar is why IE is crashing so much." They will usually remove the offending program, but it won't actually fix the problem. They will then have to actually find the real solution, and then go back and say you still can't have whatever app they removed still claiming it was the problem. They will not let you have your old application back, even though it was never causing any problems.
The second excuse will be used to explain why they won't let you have the program even before you've installed it. They will insist that every program you want is a security risk.
This is not to say that there are not legitimate reasons for the IT staff at your work to deny you from having no other programs installed except the ones they provide. A good security policy, in my opinion, is to set up an employees computer in a manner that is consistant with their work duties, and then lock it down very tight so the user does not have access to add or remove software, or make any system changes. However, these reasons should be properly articulated using facts and valid arguments, not scape-goats and non-truths.