How recently did the most recent rule implementation start and what does it say? We didn't have these luggage issues when we flew to Europe late December, also with LH segments. I would hope to see some kind of consistency between outbound and return. Does it make any sense to tell a customer they are free to take up to 140 lbs of stuff with them but can only return with 50 lbs or less without incurring additional fees? I was under the impression that the Most Significant Carrier rule applied in this kind of situation?

From the LH website:

Special baggage rules for travel to/from or across the USA:

Under a consumer protection law issued by the US Department of Transportation, the following baggage rules apply to flights to/from or across the USA:

1/ Consistent baggage rules apply to all parts of a journey to/from or across the USA. These include the free baggage allowance, the dimensions and weight of each of your bags, the level of any charges that may be incurred and hand luggage regulations.

2/ The baggage rules set by the airline which is listed first on the ticket are those that usually apply here. If Lufthansa is listed on the first flight segment of your ticket, then Lufthansa’s baggage regulations will apply to your entire journey(1) — both your outbound and your return flight.

3/ However, any airline which is listed as the first carrier on the ticket is at liberty to transfer the baggage rule stipulation to another airline in the itinerary (the so-called ‘Most Significant Carrier’). In such a case that other airline’s rules would then apply to the entire journey.

So, UA is the first carrier on my ticket; it's also the Most Significant Carrier on this itinerary. And yet, my current confirmation seems to give me the baggage allowance set by LH, not UA. Puzzled...