I hear people like to quote rule 240. IIRC, 240 was a rule that existed pre-deregulation, replaced with whatever conditions are in the airline COC that you are flying. It doesn't technically exist.

Semantics aside, UAs only true obligation under the COC is to rebook a passenger to their destination on the next available flight on UA. It's been a while since I've actually read it, but says that they may rebook an another carrier - i.e. they can, but aren't required to. Not sure if there is a *A rule that indicates something more stringent. At least in North America, most COCs are going to be similar on this.

That said, I've never - either pre- or post-merger - been refused rebooking on another carrier - either * or non-partner, when there has been a major delay, cancellation or even when a delay would lead me to miss the last connection of the night, even when it was ATC/weather. I've often had to suggest it and specific flights, but I've never been refused - either MX or WX. UA has to pay for those seats on other carriers, so probably doesn't like to do it if they can get away with not, but they do do it.

A couple of guesses: One is that you got an agent that is more willing to book on another carrier than others. Another is that most are going to line up in the same place - at the gate they came from or a CS counter, if there is one. They can only rebook so many people, and it takes longer now than pre-merger. So its possible that people weren't able to get to the agent and have the rebooking done in time to get on the flight. Many non-FFs probably don't even realize another carrier is an option, or wouldn't even think of asking for it, so combine it with an agent who defaults to rebooking on next available UA flight, and it probably isn't that surprising.