why does my dell venue pro power on by itself when charging the battery?

Jan 18th

My 1st day with my DVP and it's a blast. So far so good. After a full day of downloading apps, setting up the device and actually using it for a telephone the battery needs to be charged. I turned the device off (held down power button to "Goodbye!" ) gently set the device on the counter with the electrical charging plug in the base, and then the Dell logo/Windows Phone logo appears and the phone powers up automatically. Replicated this several times in fact.

Is this a design feature?
Are you only suppose to charge the phone when it's running? When I unplugged the device it actually remained off... but seems kinda odd that I can't charge it overnight with the device power off.

Thanks for any advice.