Law societies are also private organizations and have a right to freedom of association the same as as TWU does.  And of course freedom of association entails the right to exclude those who do not agree with your core values.

Certainly governments accredit schools based on criteria of competence, generally speaking.  But law societies are not government, nor are they a rubber stamp.  They are actually obliged as a self-regulating profession to establish their own rules about who they sanction based on their own fundamental principles.  In this case, that includes fundamental principles like equality under the law and fairness without discrimination.  Legal competence is only one part of the criteria that law societies consider in deciding who gets their sanction -- ask any lawyer who has been disbarred on moral or ethical grounds.  

I would say TWU is seeking the whole cake -- that they should have freedom of association but secular organizations like law societies shouldn't.