As a person who is critical of police and their tactics, I have to say I agree. I just had this exact conversation with my brother. This is how I would see it play out.
The first line of responsibility is that of the driver. it seems he was primarily responsible and regardless of how his prisoner was loaded it's his responsibility to ensure his safety and he should take the brunt of the blame for what happened.
The officers that loaded him into the van failed to secure Mr. Fray and are certainly in breech of department policy. Assuming that's all they did, involuntary manslaughter seems pretty steep to me. I'm not certain where the assault charge comes from, but if it's as a result of injuries sustained after the van leaves, then I'm not on board with that either.
As for the Sargent and Lt., they are superiors and if they knew Mr. Gray was not properly loaded per department policy then they should be severely reprimanded, but again, I'm just not sure that inv. manslaughter is appropriate here.
Now, if anyone has seen a charging document for a man who commits a robbery, it's not unusual to see a laundry list of charges, most of which disappear and, given the evidence, the worst 2-3 (that can be demonstrated via evidence) stick. So I have little doubt that much of what we're seeing today is tossed out before this is over. I hope there isn't anyone that hopes that they will be charged with all this stuff, because as a critic of police tactics I doubt that's what going to happen, nor do I think, at this time given what I know, that it's right.
I wouldn't be surprised if the severity of the charges and the liberal manner in which they were doled out had something to do with appeasing the natives, but as I said, I don't think it's all that unusual to "throw the book" at an alleged criminal and see "what sticks".
I should add that failure to render/ request assistance is something that some or all of the officers could be charged with, but I have no idea specific scenarios for each.