Why Does Undoing Items Purchases Also Undo Health/Mana Regen?

Serioiusly it's pretty annoying to recall when very low, buy an item, heal up then decide a different item is better only to have to wait to heal up all over again. It can make a big difference if the enemy is siegeing one of your inner towers or going for baron if your return is delayed by the 10 secs or so it takes to heal fully. I'm sure this wasn't intended, right?
Steps to replicate: go into game take damage, recall, buy any item immediately, then undo after your health is full. You will also undo the health regen.
It's happened to me at least for the last 30 undos or so, and I think it happens to everyone based on what people have said.