> I agree that it wouldn't be the best PR but it would be a relatively good way to weaken them.
Wars are won by morale. Material is instrumental to that, sure, but a relatively minor material gain (and against Russia, that's all Ukraine gains by unrestricted targeting in Russia) isn't worth a massive boost to the enemy population’s willingness to fight when that will is likely the greatest weakness your opponent has and your best path to victory. No one neighboring Russia is going to defeat them in a conventional war materially, but they have been outlasted in willingness to fight.
Wars are won by morale. Material is instrumental to that, sure, but a relatively minor material gain (and against Russia, that's all Ukraine gains by unrestricted targeting in Russia) isn't worth a massive boost to the enemy population’s willingness to fight when that will is likely the greatest weakness your opponent has and your best path to victory. No one neighboring Russia is going to defeat them in a conventional war materially, but they have been outlasted in willingness to fight.