I'm having a hard time understanding this point? What is the analogy you are trying to make here, that Russia -- the attacking side -- is showing restraint because Russian soil is being attacked? Or is it somehow that the US helping Ukraine, providing them weapons, proves Russia is showing restraint?
You do realize Russia (going back to the USSR days) has a long history of providing weapons to those fighting the US. We have Korea, Vietnam, Iraq, Jordan, etc, where Russia provided weapons -- often with military "advisors" on the ground helping. You can argue that the US had done the same, such as helping to arm Afgani rebels (and giving bin Laden his start) against the Soviet Union, though I don't recall US troops on the ground (including in Ukraine). So it seems rather disingenuous for Russia now to claim this is some type of reason to bomb the US.
On top of that, your analogy appears to be wrong. The fact is, Russian missiles have hit NATO territory (in Poland and Romania), which technically should require the US, along with NATO, to enter the war in the defense of the NATO countries hit. The US could have gone to war as NATO has been "attacked." The fact that NATO hasn't shows that the US and NATO are not trying to escalate the war.
Putin is showing little restraint, as (at least based on what Russia has done in the past) he has no cause for attacking the US or other countries. His "restraint" is knowing that any attack on NATO, particularly nuclear, will not help him meet any of his goals. A nuclear attack on the US will equally destroy Russia, or even be worse for Russia (based on what we've found out about Russian readiness and maintenance). Instead, he is attempting genocide on the Ukrainian people -- that is not restraint.
To give just one example, a
mass grave in Izium found dead civilians: 215 men, 194 women, 5 children. Mixed in were the bodies of 22 service members. Most of those in the graves died violently. We know civilians, largely children, have been removed from occupied areas in Ukraine and moved to Russia and placed with new families, their names changed (to make them impossible for Ukrainians, including their families, to find) where they are being turned into Russians. Russia appears to not only want to bring Ukraine into Russia but to destroy any Ukrainian identity or ethnicity of those that live there.