This process of restricting low-fare or any seats on connecting flights is increasing at UA, but it is rarer at others, I think. I don't really understand it the reasons for it. With respect to free tickets, I have been told that by a UA agent that she could not "build a connection" by separately booking two available flights. That is ludicrous!

Everyone should be aware that using multi-city options on Travelocity or ITN (or your own TA) will allow you to circumvent this strange phenomenon. If Sabre or Apollo will autoprice the itinerary, it doesn't matter that you booked the segments individually and does not violate any rules.