If he were to take a plea, he's saying he's guilty of a crime. I'm glad he didn't. Why should he have a record if he believes he's not guilty of committing a crime?
Well that's certainly a fair question to ask, but in our country about 80% or 4/5 cases in the criminal system are resolved by plea bargains.
What exactly is he "not guilty of?" Dennard was obviously guilty of hitting at least one person, if not two, so he's certainly guilty on some level. However in our wonderful adversarial system of US law, the defense always has a right to fight the charge based on loopholes and any lack of sufficient evidence.
The defense's argument is that the cop did not identify himself first and also that the initial student victim cannot say for sure that it was Dennard who punched him, so they do have some potential loopholes to try and use to their advantage.
Taking a plea to a lesser charge and being guilty of a misdemeanor isn't too big of a deal, especially when you're looking at felonies and multiple yrs of jail/prison time, and that's exactly why many people choose that option.
The evidence obviously comes into play along with the strength of the prosecution's case. If the case is strong (in this instance we have mixed messages from the cop testifying that it was Dennard which is strong, all the way down to the actual initial victim who testified that he couldn't necessarily say that it's Dennard which is weak) then usually the defense attorney will often recommend a plea deal, especially when it guarantees not having to gamble on a jail/prison sentence.
Anytime you choose to pass up a plea deal you automatically gamble due to taking it to trial, where the punishment is way more severe if you are found guilty.
In this case, we don't know yet whether it was the prosecution's decision to not allow Dennard the option of a plea deal, or Dennard's representation that pushed for an outright innocent verdict based on the fact that the victim #1 (not the cop) can't testify that it was Dennard, which weakens the case.
There is also a 3rd option that the prosecution offered a plea deal on some level (I personally find it hard to believe that there was nothing offered on any level, but it's just my opinion) that included some minor jail time, and Dennard did not want to take any jail time at all, even if it was only 90 days for example. Many times the prosecution still offers
something, even if it is still considered severe by the defense and they cannot come to terms. However in this case the charge may have been severe enough that the officer's opinon and the prosecution's opinion was that they are going to swing for the fences and find him guilty to the highest level that the law allows.