The franchises can make do. It's not like there are no players who fly-in/fly-out during the IPL. It's the players that would rather get an extra year's salary.
The thing about NZ is that this is all negotiated in their agreement. NZ aren't meant to book fixtures during IPL and in return for not contesting availability, the IPL gives money on their contracts. Whereas the WICB has constant player disputes and SLC can't manage to pay anyone, NZC have negotiated for a pretty lightweight worst-case scenario.
The problem is on England's end really, they're overbooked and can't bump the Test any later. It's incredibly spaz to be hosting the Ashes, the Champions Trophy and an additional tour in 5 months.