Well, one part of the charging problem they're trying to solve is getting cars in and out of slots quickly to increase overall capacity. For the existing Tesla owners in the US, there's virtually zero auth/payment overhead: they already have Tesla accounts and it identifies the car, so you just drive up and plug in and get billed. They also charge idle fees ($0.50/min if there are open slots, $1/min if the whole station is full) for occupying slots unproductively.
I imagine they could have added some kind of reliable payment-card reader, maybe. But then you're dealing with a whole other level of things like people trying 3 different cards until they get one to work and burning time, possible fraud, skimming, etc. You also have new hardware and failure modes in the charger itself (the reader, the screen, the buttons, etc), whereas right now they're featureless, just a tall plastic thing with a charge cable on it, and it's charging to a known card pre-registered in the account.
It seems a reasonable tradeoff (at least in the US, to me, at this point in the industry's evolution) to stick with the system they've got for simplicity and speed, and ask for a Tesla app install. The only other open charging network I've tried in the US (Chargepoint) also needs an app install and account.