I don't think this count as open sea. The rule is 12 miles from the coast (12 nautical miles btw, i don't know what it is in freedom units). i'm pretty sure the strait is narrower than that at the place where the toll is paid (if you count both side, i.e less than 24 miles Between Oman's peninsula i forgot the name of, and Hormuz/Qeshm islands).

So basically, Iran say "here, you have to pass through our or Oman's waters, we will let you, but please pay a toll for the derangement, that we will share with Oman."