I also read that it was UA groundstaff who reported the pilot.
Can anyone tell me why they would do that rather than just preventing him from flying?
Because drunk pilots need to find new careers? Presumably the ground staff was more worried about keeping passengers and crew alive than it was about saving this intoxicated pilot's job. The idea is not to cover for a drunk pilot, but to remove him. Those who cover for pilots who are not fit to fly are part of the problem, not part of the solution.
An article in USA Today claims that only about a dozen U.S. pilots are caught intoxicated by alcohol each year by authorities. The aviation industry can very easily do without these pilots. Unfortunately, for every one that is caught, there are probably many others who fly intoxicated. Still, abuse of alcohol is exceptionally rare among pilots, many of whom don't drink at all, just to be safe. If all pilots who insisted on flying under the influence of ethanol were removed from their jobs, plenty of other pilots would be instantly ready to step up and take their places, and they would be more than willing to stay drug-free for the privilege.
If one beer is enough to be over the limit, then why not just skip the beer and have a soda pop instead? If taking this drug is so important that a pilot is willing to risk his career to indulge his desire for it, he has a serious problem and shouldn't be going near a flight deck at all.
Yes, there are many other things that can impair pilots. But that simply means that these other sources of impairment need to be addressed, too—it does not mean that impairment by alcohol should be tolerated. The idea is to increase safety, not reduce it. Additionally, a drunk pilot has voluntarily impaired himself, then has voluntarily chosen to fly unsafely. That's a bit different from someone who has been excessively fatigued by inappropriate work schedules and risks retaliation by his employer if he dares to admit that he is unfit to fly. In both cases, the pilot needs to stay on the ground, but in the first case, the pilot is entirely the source of his own problem, which he could have avoided without any effort at all.
If you don't wanted to end your career because of alcohol abuse, don't drink. If you cannot stay away from alcohol long enough to be at zero when you fly, despite the danger of killing yourself and others and the danger of destroying your career even if you survive, then you have a drinking problem, whether you are willing to admit it or not. If you are so reckless that you are willing to intoxicate yourself despite the risks, then you have personality issues that are incompatible with the piloting profession, and you need to find a different line of work in which your disregard for safety will not be a danger to others.
The reputation of the entire flying profession is being threatened by a drunken few who never should have gone near an airplane to begin with. It amazes me that so many people jump publicly to the defense of this handful of losers; it only worsens the damage to the image of professional pilots everywhere.