Share buybacks are a capital return to investors. This is a sensible and desirable thing to do in a lot of macro environments. ZIRP may increase the instances of borrowing money to do share buybacks, just like it would increase the propensity to borrow money to do any other valid corporate activity. There were share buybacks in the 80s and 90s when interest rates were quite a bit higher than today.

Adjusted for inflation, there were more buybacks in 2000 than in 2015 and interest rates were quite a bit higher in 2000 than in 2015.