I can't speak for the specific United/PageOnce issue, but the general situation here is one that I've been involved in from both sides of the fence, so...

In general, there's a number of reasons that websites like United want to block "aggregator" services like PageOnce. In no particular order :

* Additional load on websites. PageOnce and similar services work by logging into your account at regular intervals (normally once an hour/day/week, depending on the service) and scraping your information. As more and more people sign up, that load can become significant, to the point where some banks are reporting that over half of their internet banking traffic is coming from aggregators. You don't have to search too far in FT to find people complaining about how slow united.com often is to pull up a reservation - odds are at least some of the reason it's slow is due to the load that aggregators put on the website.

* Password Security. In general, these services work by you giving them your username and password. Not only does this likely put you in breach of the Mileage Plus T&C's, but it creates a situation where there exists what could become a very large list of Mileage Plus usernames and passwords existing at a 3rd party service. What happens if/when PageOnce gets hacked and your miles get "stolen" - who's to blame?

* Security/Fraud detection. Many websites have systems in place to detect things like people attempting to guess passwords, but these generally lose at least some effectiveness for services like PageOnce as their traffic patterns are different. eg, you might block access from an IP address after 5 failed logins - but how do you do that for an aggregator? I'm aware of at least one occasion where an aggregator (not PageOnce!) was used as a part of a brute-force passwords attack.

There's also a number of lesser marketing/etc issues (if these sites result in you visiting united.com less then they lose marketing ability, etc) but these are normally secondary to those above.

This whole concept has become a huge issues for numerous companies over the past few years. As a rule, the banks hate services like these, however they put up with them because they don't want to risk losing customers if they block them. I'm guessing United have decided that it's worth (small?) the risk of losing passengers to block them. It's become such a problem that at least 2 of the major banks are planning to charge aggregator services to access data.

As annoying as it might be, I can't blame United for doing this.