To be honest, I believe that the "Man managing the desk" has the right to move you along, using the the minimal force possible, By grabbing you he can claim that he was protecting him self, from you possibly lashing out and using your "Teenage" Anger on him, Least to say I've seen the damage that a skateboard can do to people in a fight scenario while you may believe that you was "only skating in the garage" to them you can be damaging there profit margin's by diverting THERE customers else where.. While if you was reacting to being moved on in a positive way, I believe you could have avoided the confrontation.
By the way, I've been a skater, although a Rollerblader I've had my fair share of being moved on, for the most minimal thing's such as skating home though the subway, by a community support officer for "endangering" people around me (even though it was just rolling down the pathway) being stopped outside a skate park for being on private property etc, but i've avoided confrontation by being polite to those people who have higher authority around me, use this as one of your life lessons and next time you're asked to move on, follow the rules of the establishments
Lesson learnt?