There's no excuse except the stitch-up in the wholesale market and some kind of fantasy dreamland they call UFB.
The current wholesale costs for ADSL and phone services are regulated with pricing set by the Commerce Commission.
Population density and economy of scale are significant factors in the price of wholesale network services, if we wanted to screw over rural users it's possible urban prices could be a lot lower and rural a lot higher but since they generate pretty much all of our export revenue the averaging out of price we have now is better for the country.
High wholesale costs are one factor out of many.
The wholesale costs are probably too high I agree, however both prices and sizes of data caps for retail plans have been and are still improving.
There are multiple ISP's offering un-metered plans now and larger more conservative ISP's are offering larger caps.
UFB is going to make a signifcant difference on the cost of data/size of data caps.
But I digress: just because you pay less for Slingshot, that doesn't mean you should expect to be offline for extended periods, encounter inconsistent speeds or poor customer service. If you do, then you can fully expect that you'll be ripped off with everything you buy.
You have to have a special way of thinking to believe a budget ISP is going to have the same quality of service as a premium ISP.