They are both very large deterants, and people on TV rather than decide to join in tomorrow might see, the idea of joining in tomorrow, just getting their asses shot, knocked to the floor, soaked and arrested.
You couldn't be more wrong in suggesting that rubber bullets and water cannons wouldn't do anything. Tear gas, it will back people off when required, rather than watch from 20 metres as people rush into a store to loot it because they don't have the man power to push them back, a tear gas shell and people won't be easily able to loot the place. Its all about making looting not worthwhile for the people coming out, making it hard, making it work, making it hurt. Right now its easy, its low risk and high reward, when it becomes high risk and low reward, numbers will drop quickly.