They only do that in markets where they're not established. It's just an extremely high marketing budget that gets spent on discount codes. As soon as Uber has a foothold the codes disappear and people have to pay for their rides.
Ita no different to the $10 PayPal gave people for signing up or the $7 of BTC Coinbase give to referrals really.
The fact that Uber are still bleeding money after being established in most markets they still operate in ( others were sold), would indicate otherwise.