How worrisome is this? You could make a narrative that large stable companies are borrowing money at low rates, buying back shares of their own stock, causing the value of each share to proportionately rise, allowing sellers of that stock on the secondary markets to command higher prices, and then those sellers spend that money, either on shares of others stocks or consumption or actual business investment.
So the net effect is that low-cost debt flows into consumption and/or business investment in other unrelated companies. I think there is a good case to disapprove of private equity firms who leverage businesses to the hilt in order to collect fees and then dump the stock in some form, a la Toys R Us, etc., but this seems slightly different, although tangentially related.