When you steal something from someone's hands and walk or run away, it is considered a strong armed robbery. I'm not making up the law. That is what it is. When you grab a security guard who is stealing your property, it is generally excused because you are entitled to use reasonable force to recover your property when you are robbed.
If you disagree, then run for office and change the law. Make strong armed robbery legal. Then you can join the #blacklivesmatter movement.
You have no idea what right a guard had to take something under the law or not, but I can assure you violence against the guard is illegal. Jumping a security guard because you want your sign back is not self defense.it is assault.
When a cop does something you don’t like I strongly suggest you do NOT jump him.you will get more than a slap.