Very good points about battery health & longevity, which I'd definitely prioritise over gaining a few mpg.I don't know Kia's thinking yet will offer an anecdote from past experience with a HEV, a Civic Hybrid. I owned one and suffered two battery failures, not because of the below but just poor cell balancing/overall design.
Honda's 2nd gen Civic hybrid initially allowed a charge range of IIRC 10-90%. People liked that and it made the most of the smallish battery, providing good mpg, usually beating the EPA estimate.
Then Honda started seeing too many early failures of these batteries, often replaced under warranty. They pushed a software update that reduced the range from again, IIRC 30-80% or maybe less. It was mainly about the minimum charge level it'd allow before starting the ICE. I think it did often go to 100% in a charge situation. Anyway, problem solved, or made better - except now there were a lot of peeved people for the resulting lower mpg and a lawsuit I'm pretty sure. It's easy enough to search and find more about it on the net.
Consider Kia's 10 yr/100K mile hybrid system warranty. Batteries do still fail and Kia has to control their replacement rate, or it's bad news in more than one dimension. The most effective way to do this, besides a solid battery design including good cooling, is to restrict that charge/discharge cycle range and keep the average SOC in the middle. I won't here go into chemically why this is, but am just positing a likely reason they did it this way, and why they're not going to change it.
For what it's worth, after what I went through with my Civic hybrid, I'll take a little less (P)HEV range any day for the relative peace of mind the battery is being "taken care of." Everything I see so far tells me Kia's engineers know what they're doing with the BMS.