Because it's harder to pick and choose trial evidence that exonerates the cops of their malfeasance unless you have something evidencing that they told them to stop before they fired their gun, regardless of whether they actually said it before firing their gun.
I don't think in general the ACAB crowd means "no police". It means better training, independent oversight, and a division of roles (e.g. social work, welfare check).
Basically actually having accountability would go a long way towards weeding out the root problem
Wounded people can still shoot you back/stab you. They may even be more likely to do so due to the shock and adrenaline rush. Also the fact that one has been shot indicates that they probably had something in their hands in the first place or were unwilling to show that they didn't.
After they've been shot, lethal force has already been determined to be necessary. Cops don't stop until they stop moving or they're making it very, very obvious they surrender. (side note. Having something in your hand is not justification for lethal force. 'I thought it was a weapon' as an excuse has worn too thin. but they'll say other bullshit too like, 'they had an aggressive posture' and 'were verbally combative'. )
Also, people typically fall back on their training in stressful situations, and they're taught to shout things. like "show me your hands" ... a lot. they may not even be entirely aware of why they're shouting it. or even what they're shouting. (Cops have been known to yell some freaking dumb shit after shooting people. You see the videos of them yelling "Get on the ground" when the dude is already... on the ground? just saying)
If the person is able and willing to surrender after being shot, showing their hands (empty of anything they may have been holding before) and following instructions means the police can start first aid and bring in EMS sooner. The ambulance doesn't come in until it is safe to do so which won't be until after a search is done and handcuffs are applied. Specific details beyond that and exceptions to the norm would depend on local policies.
Assuming there was ever anything in the victim's hands to begin with. Plenty of cops will go on to deny the person emergency treatment under the guise of the individual being "too dangerous"
Edit: To anyone daring to question the veracity of this statement, just go ahead and watch the last moments of this man's life then take a good long look at yourselves and think about just what you're trying to defend here. This is nothing but murder. Decided on by an overly-sensitive EMT and endorsed by six different cops. Be ashamed of yourselves.
Just a guess, but I'd think it'd be based on fault. If it's a justified shooting, they'll probably tack it into the suspect's fines. Ideally, if the shooting isn't justified, the govt would pay, but I wouldn't count on it.