Mate it's not strange that you were not cuffed and taken back.
If people are pleasant, polite and honest then they get treated well. I don't mean honest as in makes confessions - if people don't want to confess to me then I'm fine with that but I mean not outright bullshitting.
Were you only charged with possess? I don't know what the weight cut offs are in WA. 1.75g is a lot but I don't think it'd be into supply category. You had no scales etc on you did you?
You're getting a fine. MAYBE a suspended sentence but I'd be surprised. Go and do a drug rehab course. Stop using meth. Especially stop carrying meth when you have your son with you - that's appalling.