Why does France ban statistics on ethnicity and crime?
Because being "French" is independant of colour or background. Where a French person originated from or where their parents where born should have no bearing on them as a French individual -so there is no need to collect statistics on that.

From a practical (and also Anglo-Saxon) perspective, this makes no sense -there are uncountable demographic, social and economic factors which transcend nationality. But, the French establishment's view is that to be French is itself a transcendental status (above demography etc), aquired by immigration and assimilation or birth. I'd go as far as to say that the French are suppremely arrogant about the superiority of their brand of liberty over the rest of the world's (read Anglo-saxon concepts of it).
That's kind of absurdly funny, but distressing at the same time. They don't check and see if immigrant groups are committing more crimes because...they are proud of their brand of liberty and don't want to "soil" it?