This should be illegal.

What should? Stock buybacks? Why?

It’s usually done to basically pay to increase companies own stock price. They often do it because CEO and executive pay is based on achieving certain goals such as stock price. However, every penny thrown at investors in a buy back is money that could have been used to weather a downturn, or increase employee pay or simply reinvested in the company itself. This often leads to companies then requiring government bailouts to continue functioning when say a global pandemic hits. The Plain Bagel has more detailed video on the ups and downs of stock buybacks on YouTube.

Stock buy backs are a tax efficient way to return money to stockholders.

The real issue is when companies borrow money to do this.

I would say stock buybacks are already a real issue, doesn’t matter if they’re borrowing the money or not. If companies had to actually invest in their employees instead of their rich shareholders a lot of issues in our society today would be lessened.