You don't mention your discipline, so this may be off base.
But in the sciences, you are largely being paid to forward the goals of the PI and the lab, and being asked to help with another students project is absolutely par for the course.
It would not at all be considered blackmail to get the sort of email you cite in any lab I'm familiar with, it would be considered a reprimand to a graduate student who's not living up to the expectations of the PI and department.
The latter part, the request that you spend at least a few hours in lab every afternoon is also worrying to me, as most PIs would expect (in a discipline with labs) that you're in at least a normal 40 hours a week, either working on your projects or helping with general lab upkeep and maintenance/training junior graduate students/helping with other projects.
Is it perhaps possible that there's a rift in understanding between you and your professor about exactly what is expected of a PhD student?
