This is not categorically true. I have had instances where the next onward flight is more than 4h00 away (same day) and a transfer won't auto-price. I don't know if this was a somewhat weird case of if the longer break must also be "overnight".

UA.com will of course show you "longer connections" but in almost all cases for a domestic itinerary this is because there is a fare break. jsloan is also correct that the relevant factor is that the fare component, not the connection city, is domestic.


To try to clear up semantics:

A transfer is a connection made on the same fare component. Generally speaking, transfers do not constitute a break in travel and the airline as no obligation to fly you to that point (e.g. if transferring in SFO on EWR-SFO-SYD, you could be involuntarily re-routed EWR-LAX-SYD with no compensation). Generally, transfers must be 4h00 or shorter on domestic fare components and 24h00 or shorter on international fare components.

A stopover is a break in travel in the middle of a fare component. It may be of unlimited length, subject to the validity of the ticket and any maximum stay restrictions in the fare rules. Domestic fares generally prohibit stopovers, and many international fares require a fee to add one. Because there is a break in travel, if your ticket is constructed with a stopover, the airline has a contractual obligation to deliver you to your stopover point.

A fare break is a break in travel where you are changing fare components. Again, you can remain at a fare break point for any amount of time subject to ticket validity and stay restrictions. Technically speaking, your "destination" is usually just a fare break point on a round-trip itinerary.