Bikes sold in the US need to meet DOT requirements. There is no specific language that I know of preventing LED turn signals from being installed, however there is language related to the minimum surface area of the lens. As the LED units fitted to EU bikes are much smaller they can not be used here.
Please note that although the EU units will mount up to a US bike they will throw faults as their current draw is lower. Most LED turn signals will throw these faults.
To avoid them you need to match the stock turn signals current draw.
Each stock signal draws .65A @ 12V = 7.8W
Lets say the LED turn signals you selected draw .25A @ 12 = 3W
Taking the difference of the current draw (.65-.25 = .4A) and plug that into Ohms law (V=IR) you get
12=.4*R solving for R you get 30 Ohm. In this specific case you would need a 30 Ohm resistor that is capable of dissipating 5W as heat wired into each LED signal.
Note:
For left and right side male/female connectors are switched, so you need two male and two female for a complete set. Right side indicators need female, left side male ones.
Your part count is correct but the rear turn signals plugs are swapped when compared to the front. So FR will match RL and vice versa.
Then there appears to be a refurbishment kit from Energica of New England to convert the incandescent inlay of the indicators to LED. They even have a how-to-video on Youtube. Might that be a solution for you?
The EoNE turn signal kits are a very nice (all be it expensive) upgrade that used the factory turn signal housings. The front kit also doubles as a set of white driving lights, the rear kit adds red lights that turn on when you brake, both are stupid bright.
Rizoma also makes some very bright, very small LED turn signals.
