He is not seeking to justify the Satanic mills, these symbolize the degredation that we should try to remedy; 'Jerusalem' is an ideal end (symbolically expressed, nothing is stated about its precise nature, different people could imagine it in different terms), the polar opposite of the present fallen world:
I wander thro' each charter'd street,
Near where the charter'd Thames does flow,
And mark in every face I meet
Marks of weakness, marks of woe.
In every cry of every Man,
In every Infant's cry of fear,
In every voice, in every ban,
The mind-forg'd manacles I hear.
How the Chimney-sweeper's cry
Every black'ning Church appalls;
And the hapless Soldier's sigh
Runs in blood down Palace walls.