With GAAP reporting, stock comp goes on the income statement and affects the bottom line. So it has a visible effect for investors, but no one cares about P/E anymore so...
GAAP reporting requires that stock compensation is written off as an expense. It does not reduce the company’s cash, but it does reduce the company’s earnings. Hence it changes the price to earnings figure.
If it doesn't cost the company any money, why is it defined to affect the company's earnings?