Tech people live in a bubble. Because our skills are in high demand and there's a pretty strong natural aptitude barrier limiting the number of new competitors, our experiences competing in the tech bubble can be very misleading.
It's a dog-eat-dog world out there. If you are lazy about competing, or trying to "go easy" on someone, you are going to get eaten. This is a hard-coded biological response to scarcity, and it's not going to go away, so we best get used to it and stop being everyone else's doormat.
Successful companies are usually fairly ruthless; they're just good at keeping it quiet. Not only does what I know about Uber's program not surprise me (and I haven't read in-depth on it yet, so this is my disclaimer if there's something more repugnant than signing in to Lyft buried under the covers), but it's downright conventional.
Refusal to compete is suicide. Naively believing that the world will reward you if you play nice-nice and opt out of decisive, competitive action is the best way to find yourself/your company on a platter at someone else's table.
This ethos from some of the other commenters that you shouldn't do something because it might make your competitors unhappy is absurd. Show a little sporting spirit. If a competitor is making intelligence available, you absolutely should take advantage of it.
I've found the guiding star to be "Is this really immoral, or have we just been socialized to believe it's impolite or in bad taste?" One should realize that a lot of this socialization is provided by people who simply don't want you to become a strong competitor, and have indoctrinated this imaginary, soft, cushiony world into everyone's brains so that it's easier for them to come in and take what they want, and their constant empty blathering draws attention away from their dirty deeds. There's no need to allow ourselves to be victimized like that.
Specifically, it's ridiculous to claim that using an API in a way that isn't intended is "morally bankrupt". That's like saying it's morally bankrupt for Walmart employees to go to Target looking for places where they can get an advantage. Far from morally bankrupt, that's a fundamental component of capitalism; how are you supposed to compete if you can't see what your competitor is doing?
Patronizing competing services in a non-disruptive way is normal. Some may be familiar with "secret shoppers", people that companies pay to go to their stores on a random day and report back on the experience. These secret shoppers are also deployed to competing stores. How and/or why is this any different?
Private and undocumented APIs have been exploited by developers for decades. My understanding is that Uber was not even doing that, but rather just accessing the normal API in a non-disruptive way to learn some useful information about their competition.