No-one disputes that protests like those seen in the Indian independence movement, the Civil Rights movement, the Iraq War opposition, the Arab Spring, the gay rights movement etc are extremely valuable and necessary.
Where people object is where justifiable protest switches to ugly mob behaviour, including things like looting of innocent business operators, indiscriminate violence and vandalism, etc , just like what happened in Tottenham.
Mob behaviour, by definition, is where people stop thinking critically about issues and blindly follow whatever their peers are doing. This can never be beneficial. Independent, critical thought is essential for positive change to be effected.
The problem with the modern Social Justice Warrior movement is that it is counter to the interests of true social justice, as the people who need the most protection are the smallest minorities, and these are the people who lose out the most when everyone is banding together in mobs and deliberately shutting silencing debate and individual expression. Meanwhile, the "social justice warriors" are far too busy focusing on how moral they seem rather than how much real social justice they're achieving.
If your response is - as I would hope - that true campaigners for social justice are not like this, then that's ok - they're not the ones being criticised.