Where did you get the idea that the charges against Valeris were dropped? Spock is shown with her in custody at the end on Khitomer. McCoy's line "And the good news is, they're not going to prosecute" is referring to the Enterprise crew breaking into the peace conference, not Valeris.
They were perfectly clear. Spock is holding her prisoner the last time we see her in the movie. Don't blame the filmmakers because you misunderstood the ending for 27 years.
What an idea? To actually present something an audience hadn't seen before.
The line right before that is "Once again we've saved civilization as we know it." Kirk is clearly talking about the Enterprise crew, the people on the bridge with him. The people he's been on many adventures with over the last 25 years. McCoy's line, replying to him, states that they aren't going to prosecuted for the crimes that they committed (breaking into the conference, escaping Rura Penthe, going radio silent from Starfleet, sharing classified information, etc. ) exposing the conspiracy. There is absolutely nothing to imply that Valeris is being forgiven or set free. It's just a humorous epilogue along the lines of what we saw in most TOS episodes over the years. (See "The Galileo Seven," "The Changeling," "The Trouble with Tribbles," and "Mirror, Mirror," among others.)
What are we to infer from those lines other than that Valeris and her fellow conspirators were arrested and prosecuted?
This is not an opinion. This is you completely misunderstanding what's in the freaking movie.
Despite what movies constantly show us, you can still be prosecuted for crimes you commit while trying to prove your innocence of another crime. If you frame me for murder and I break into your house to get evidence that proves I'm innocent, I can still be arrested and prosecuted for breaking into your house after I'm found innocent of murder.
