Following along, the LSA rules don't allow night flying or instrument flying. Does the engine self land 30 minutes after the sun sets, or does the plane fly out of the clouds to VFR conditions if the pilot accidentally/intentionally gets into clouds. I know the answer to these is no. However, the plane is capable of either of those restrictions, even though it's an LSA. Therefore, the onus of compliance seems to be the Sport Pilot for night or instrument flying. Would not the onus to keep the speed below 120 kts be similar, if for some chance the plane/engine is capable of sneaking past the 120 kts. It's obvious an RV10, RV9, etc. would have the capability to exceed the 120 kts anytime it wished, and no reasonable argument would allow a Sport Pilot in those planes to argue they comply with the restrictions. But the LSA plane, in many allowable configurations, could sneak above the 120 kts also, though the LSA pilot acknowledges he shouldn't allow it to.
I guess the point of my rambling is many normally configured LSA planes could get above the LSA restrictions, but the FAA says configured normally as an LSA, they shouldn't and the Sport Pilot acquiesces to that compliance by adopting the Sport Pilot certificate.
It sounds like you are mixing two sets of rules.
The rules for a sport pilot, allow for piloting any aircraft designated as being light sport, or an aircraft that meets the light sport performance parameters. That aircraft can be certified as standard category, experimental amateur built, experimental light, sport, or special light sport. There are additional requirements for a sport pilot, such as not flying at night or above 10,000 feet MSL.
If a private pilot is flying any of those aircraft they can fly at night if the aircraft is properly equipped.
Then there are rules that regulate what constitutes a light sport aircraft. The basic standards are no faster than 120 kts at max. continuous power, stall speed no faster than 45 kts at gross weight with no lift enhancing devices used.(flaps,etc.), Max gross weight of 1320 pounds or less.
There are other requirements, but those are the primary basic ones.
So any aircraft that met those requirements at certification and has always met those met those requirements whether it is certified as a light sport aircraft or not, can be flown by a sport pilot.
The IFR restriction applies to special light sport only because the ASTM’s that light sport is certified under specifically prohibit it.
Experimental light sport doesn’t have that restriction. Nor does any standard category aircraft that meet light sport requirements, as long as those aircraft meet the minimum equipment requirements for IFR.
There is a good chance that this will change in the future. Just like many other rules, for the light sport have been changed over the years.