It happens when Exchange is activated. I just got the HTC 8X, and didn't activate my work email for about 2 weeks until I was able to organize all of my contacts and download my apps.
The day I contacted IT to activate my email on my phone, I was forced to create a 4-digit pin. You can control the time allotted before you need to enter it again, I believe up to an hour.
I work for a global entertainment company and often receive confidential information via email. Any company with proprietary information should retire password protected phones, especially if you were to lose it or get it stolen.
The email you send/receive is property of the company you work for, not your own, even if on a personal phone.