re: Does the Janus decision only apply to public sector unions?

The decision still notes that the government can negotiate with union to be the sole entity to bargain employment issues and be binding on all public employees.

The majority opinion just makes it clear that government cannot go the extra step to force public employees to pay fees (i.e. fair share fees) to support the union and may only deduct fees if the public employee voluntarily and affirmatively chooses to make the contribution.