They are mad because they feel Pantaleo has been made a scapegoat for following orders. (A lot of unexamined assumptions going on there.)
It's been my opinion that a reason why the 'thin blue line' is so rarely broken is because the whole situation is FUBAR, police are NOT working class & and any police 'union' is bogus (though most call themselves 'associations'). Police will support any bad apple that gets in trouble because they MUST (according to their logic) support each other because the brass will always throw someone to the wolves rather than take responsibility for their orders. The people giving the orders, in their turn, are ultimately getting their orders from the 1%.
On Wednesday, the NYPD revealed that the sergeant who supervised Garner's arrest would avoid a disciplinary trial by pleading guilty to the charge of failure to supervise, with the punishment of losing 20 vacation days. The ten other officers involved in the arrest will not face discipline, according to the city, including Pantaleo's partner, Justin Damico, who initially claimed that Garner was trafficking enough cigarettes to be charged with a felony, and told investigators that force was not used in the arrest.
The whole article is full of unexamined examples of why the police system is so rotten it can’t be reformed. Individual police officers and their supervisors and administration all have a vested interest in keeping their jobs.