Re: @Jedit - "Why can't they buy their own?"
The system already allows for BYOD. I'm a government employee, and at work I use a 23" widescreen monitor that I supplied myself. The sticking point is that if it breaks, my employer has to replace it with another 23".
In and of itself BYOD is a great idea, and in smaller businesses it works a treat. However, when you're dealing with hundreds or thousands of employees the effort required to get so many different devices talking to each other is not cost effective. You're also faced with the issue of what to do when someone's personal property breaks in the course of doing their job; they have every right to claim a replacement from their employer, and the employer loses all savings from central procurement contracts because they can't establish a standard device to buy in bulk. It's easier and cheaper just to give everyone the same.