The NUM says there was a collective bargaining agreement in place at Lonmin in 2012.
Legal teams are submitting final arguments at the Farlam Commission of Inquiry this week.
The families say the killings at the North West Platinum mine in 2012 cannot be justified.
Amcu has pointed fingers at Cyril Ramaphosa who was a Lonmin shareholder at the time of the shootings.
Lonmin defended the deputy president, saying he only wanted to stabilise the situation and end the killings.
The South African Human Rights Commission has strongly criticised the police's conduct.