It's all about risk and assessment of that risk. In regards to throwing people out for sitting in the wrong end, you have to look at the risk involved. If there are fans in the wrong end and they are behaving, keeping quiet and, if found out, are respectful of those they sit amongst, surely the risk is minimal. If there are fans sat in the wrong end who jump and cheer a goal, are abusive towards those they sit with or their team, they are putting themselves and those around them at risk. Its the job of the Police to assess that risk, what the potential implications are, and reduce that risk.
It's the same in this instance. From where I sat (i the next block and the same side) the Police were walking up the stairs to retrieve a banner (rightly or wrongly... thats another discussion). The tension was already ripe in the air. This fella had already shown signs of drink and was obviously hyped up with the atmosphere. He literally clocked the Policeman without his numbers and started screaming over at them. The officer who came to speak to him eventually calmed him down (reducing risk) which is where the cameras start to pick it up. Now from then on in, the Police became very heavy handed, particularly the one who put a forearm around the fella's neck. That is quite simply not on. The fan himself though, is not completely blameless though, as he essentially created that situation by displaying aggressive behaviour over something that was trivial. He was being anti-social and abusive towards the officers and around minors, again, putting himself and those around him at risk.
The riot that ensued afterwards was just pure stupidity. Was it the fault of the police that someone fell down the stairs in an effort to attack them? Of course not. Was it his own ridiculous decision to come rampaging down steep concrete football stadium steps to get involved in something that, in any other game, under any other circumstances, wouldn't have blown completely out of proportion.
There is no blame, so there can't be a claim. It's just a series of bad decisions and unnacceptable behaviour.