Unions owe their power to being able to coerce workers into contributing dues even against their wishes. Let workers have a choice whether or not to join a union and many will choose not to join. This has nothing to do with collective bargaining. Unions still have that right in right to work states if a majority of workers vote for union representation.
This just means that now unions will be forced to provide something of value in return for their dues if they wish to retain members.
Kind of like government and taxes except taxes are generally a much larger fraction of one's pay.