Likely, the DA felt it simply wasn't a case he would win with a jury. Just because the DA drops charges, doesn't mean the person didn't do anything wrong.
He didn't have a right to be there though. It was not "public property". It is a private business building open to the public except it does have areas that are not open to the public. And even businesses open to the public can ask people to leave. This guy went straight to claiming racial discrimination. Why? Because that is in fact what he does, writes/sings songs about racial discrimination (imagine that). No, signs don't need to be there if someone is telling him multiple times that he can't be there, an official of the building in fact. If the security guard is wrong, that is dealt with somewhere else. You complain to management, you don't not listen to the security guard, who is a representative of the owner/manager.
His actions are what caused him to get tasered, actions that could not be seen on the video. Actions described by the police on their reports however. Cops do get to taser people who are resisting arrest, as this guy was. That is not an abuse of power, it is controlling the situation.
Bull. The DA did nothing but drop the charges, months after they were filed. Most likely because he felt he couldn't win the case or had a lower than he liked chance of winning, so figured the charges weren't worth pursuing. It didn't have to have anything to do with whether the DA actually believed the guy or the police was right in the matter, only what he felt he could or couldn't prove.
And police are not held to the same level when it comes to what they need to prove to either detain or arrest someone, heck even for use of force. Just because the DA didn't feel the case could be proven beyond a reasonable doubt, doesn't mean the police wouldn't be justified in their actions to detain/arrest this guy, including tasing him.