Because it means the management is not investing the proceeds in the employees and the operation, they're buying stocks back to increase their own compensation. This from the Communication Workers of America:

Stock buybacks are when companies buy back their own stock from shareholders on the open market rather than investing in workers or equipment. When a share of stock is bought back, the company reduces the number of shares left in the market, which raises the price of remaining shares. Company executives have every incentive to buy back stocks, since most of their compensation derives from stock and a higher stock price makes them personally richer.

It hasn’t always been this way. Until 1982, buybacks were considered a form of market manipulation, but a Securities and Exchange Commission (SEC) ruling that year gave companies free reign to buy back stocks.