In the immediate moment they do not have to tell you why you're being arrested. Generally, they will worry about taking the person into custody first. Nor do will you have your rights read to you unless they plan on asking you testimonial questions. (I.E. they can ask you your name and routine booking questions)
As for the use of force here, I believe it was justified in terms of it not being a criminal action. You have to do what you have to do to effect an arrest. While the choke hold is banned by NYPD doesn't mean it's illegal, that is only a policy. Additionally, his death was caused, largely, due to his health. Please watch the video and hold your breath for the time he is in the choke hold. It's not long. If you don't have the health to fight the police then don't.
With this in mind I don't agree with the choke hold, it's not something I would use. But within the parameters of the law it is not a crime. And I expect the family will get a very large sum of money as they will be compensated for their loss and a large punitive damage will be awarded to discourage officers from using choke holds or what may be used as "excessive force" and also with all the pressure from the media i'm sure the damages will be inflated.