Think of it this way: You can divide just about all instrument in the aircraft into two types: Those which provide you information with will not extend the capabilities of the aircraft, ans those which could be used to extend the capabilities of the aircraft. Instruments like airspeed indicators, tachometers, and oil pressure indicators, though required by regulation, with a few exceptions and grey zones, cannot be used to enable the aircraft to do anything it could not, were that instrument not have been there.
The instruments which are most commonly installed to extend the capabilities would be the artificial horizon, and radio navigation instruments. These enable you to fly out of visual reference with the ground. whic is somethong which you could not do were they to not be installed.
Therefore, though the presence of the instrument itself in the aircraft may not violate operational regulations, how you use it might. Infra red, or synthetic vision certainly fall in these catagories. To some degree, this is borne of the concern as to how your flight would continue safety immediatly after it failed, were you to be relying upon it. Would the failure even be recognizable?
This is the senitment which results in the "Not for primary navigation" placards on some systems. To remind you that you are not allowed to make your flight dependant upon it.
Infra red is a great safety nice to have, but otherwise should not be expected to be used to extend operational capability beyond what it would otherwise have been.