Rappelle: Had the same problem. Didn't realise they had dropped their price. A notification would have been nice, and I think that's where they've gone wrong. Called to request fibre, told the price for adsl would be $75 now - would have been nice to know that a couple months back.. But hey. I was signed up for the $90 plan, I can understand that they would need your permission to switch plan? It's like if they increase the price of a plan - you still keep the same price you're on until you request to change plan.
Correct me if I'm wrong, but the $90 dollar plan includes VoIP phone line and $75 one does not. You still will come 4$/month cheaper if you add $11 to $75 for VoIP, though.
Edit: Sorry, did not read the whole thread before replying. To add something, here, have the link to iTalk: https://www.slingshot.co.nz/italk/overview
Btw, I personally do not see a huge issue with Slingshot in the situation described. If you want to get better deals you would be checking prices and services of internet providers regularly anyway (not just Slingshot) so you would be aware of a price drop. If you don't care that much to check, well then you keep paying the amount you have agreed to pay for the service you subscribed for - there is absolutely nothing wrong with that.