Their businesses are profitable, they just aren't booking profits. Odds are eventually they'll simply use free cash to buy back shares instead of reinvesting it. Which is why Amazon attracts quite a few very sophisticated long-term investors, and why they command a relatively high valuation.
Well, it's possible to have free cash without being profitable on paper. For instance, remember that depreciation is a major expense for capitalised assets (such as fulfillment centres, data centres, computer equipment, real estate, etc.) but isn't actually a hard cash expense, just a formal expense. The hard cash expense came at the time that the investment was made, i.e. when the data centre or what have you was actually bought and paid for.
So, the difference between that "virtual" expense and actual cash is one accounting category that free cash can come from. There are others.