The US does allow clear side markers, however, the US Verison IS-F does not have them because it is not required here in the US as in other parts of the world.
As far as the cooling feature in the seats, they were not installed not to save weight, but for the simple reason that the new IS-F's seats could not accomidate both the heated and cooling systems in them.. One of the first things I noticed about the vehicle before it was even introduced to the public and I spoke with on the IS-F team. I mentioned that I thought it made more sense to have the cooling feature of the heat since most of the time, you will be driving the car in warmer weather, not cold weather....