That's correct, Hvy. It sort of makes sense. HECO's got a big generator(s), and they regulate the output to match demand. So while the sun's out, all the PV systems humming right along, and the main power station is throttled back. Then, when the clouds come out/sun goes down, HECO has to spin up their generators to crank out more electricity in order to make up for the electricity that's not being generated by all the PV systems. If there's only a small number of PV systems, it's not a big deal; but if, say, half of the electicity was being generated by PV's, it ends up making large fluctuations in the amount of power that's needed from HECO. That makes the grid unstable, so before you go over 15%, you have to do a study to make sure that your PV system isn't going to make problems.
Naturally, no one is required to buy electricity from HECO. If you replaced all your shingles with PV cells, and stuffed your attic full of batteries, nobody could say 'boo' to you about it.