Putin is deliberately isolating conflict regions from each other and from the bigger picture, somehow hoping that no single attack is worth it for NATO to actually retaliate in any form. So far, as long as no NATO countries are in immediate danger, this works extremely well.
An example could be nuking a large staging area or aircraft carriers. There's nothing irrational about it, once war progresses this far.
The premise is that when the Soviets decided to invade they'd lob a few of these at their forces and pin them down for a little while, so the rest of NATO could mobilize.
It seems that even at that time, the US wanted to keep the conflict nuclear-free as much as possible.