You can break that question into 2 parts.

First, the 3.9B figure appears to represents all RSUs vested up to this point, aka many years worth ("(2) Q2 2019 includes $3.9 billion of stock-based compensation expenses, primarily due to RSU expense recognition in connection with our initial public offering"). That's why its slightly misleading - its many years worth of stock comp for employees which is all getting recognized this quarter (Happens in pretty much all the big tech IPOs)

In general though, its very reasonable to associate that as an expense, which is why GAAP requires it. Where that gets complicated is that its not an immediate cash expense to the company, hence some fudging around with non-GAAP accounting which is seen by some companies.