The mobile data situation for tourists is not good, but this chart and story don't accurately represent the US market for mobile data.
It says it's for prepaid in the title. Its not inaccurate just because it does not measure what you want it to measure.
When I roam from Canada to the US, the incumbents charge something like $1 / 4 KB (yes, KB, not MB) of data for anyone on a post-paid plan, and deny data roaming to pre-paid users. I keep a T-mobile SIM around just for when I visit, even though I only use it for a few months every year.
If the cost of data is the same for 500 MB as it is for 6 GB (most prepaid plans I could find last time I visited were either 100MB with huge overage costs or 2GB+) then the relative expensiveness of the US goes down as the amount of data used increases.
I'm not saying that wireless data in the US isn't expensive -- it is. Besides, financial ITMPs are one of the most effective ways of regulating data use when bandwidth is finite.
I agree with the statement that "prepaid metered mobile data which rolls over and represents less than %0.5 of the market is vastly over priced in the US."