Originally Posted by
Mark in CA
Had I brought the same number and weight of luggage to a United flight (which I thought this was), I would not have been charged extra because of my Premier status.
From your comment I'm assuming you booked "UA" flights (UA flight numbers) through United or one of their agents and the flights were operated by LH. If this is the case your contract was with United Airlines and their terms and conditions apply to your journey. Lufthansa's rules are irrelevant here - your contract was with United.
The US DOT, in 2009, took the opportunity to remind the airlines that for code-share flights the operating carrier's rules do NOT apply - the contracting carrier's rules apply regardless of whose metal is being used. The DOT informed the airlines at the same time that it wasn't enough to say something like "In case of code-share flights the code-sharing airline's rules apply" - they had to inform the passenger of the rules that would apply. The "guidance" can be found
here. (MS Word Document from dot.gov website)
The airlines don't like this ruling (which is an obvious one in my opinion) because it is difficult to apply different rules for different passengers on the same flight. It seems that they 'screwed-up' with you.
I would complain to United (directly) that the check-in agent misapplied the rule and ask them for a refund. If they demure I would suggest you forward the exchange to the DOT's enforcement office.