The burden of proof lies on the proponents to demonstrate why the burka should be banned, not why it can be banned.
Wearing a crucifix around your neck is not an obligatory part of the Catholic faith but the government is not considering banning crucifixes. The burden of proof is on the ban's proponents to demonstrate 'why', not 'why not'.
A burka ban may not directly infringe a Muslim's freedom of religion but it undoubtedly infringes their freedom of expression and their right to wear whatever clothes they want to. Whether the burka is an integral part of the religion or not, it is irrelevant. On a civil level, the ban dictates what a woman can and cannot wear. This is an infringement of civil liberties and promotes the repression of women.