The short answer: Yes, this is just HD marketing to convince customers to pay even-higher Harley taxes. They have so much practice doing damage control, they've developed a strategy of trying to convert their screw-ups into sales opportunities.
Nearly every DOT4 fluid sold in the U.S. has approximately 3% of a "proprietary" rust-inhibitor mixture. Over time, the rust inhibitor is used up, and rust can start to form. As long as the fluid is changed regularly (or when prematurely degraded by air leaks, excessive heat, etc.), there won't be any problem.
As we on this forum know, HD had a problem with bad seals in their brake and clutch master cylinders, probably b/c the foreign manufacturers used materials incompatible with DOT 4 fluids. The problem would be magnified by depletion of the rust inhibs.
If HD was offering me a free flush with the new fluid, I would probably take it just in case there had been some internal damage to the ABS pump, etc.---that way, if the damage shows up, HD wouldn't be able to weasel out of responsibility by pointing to our failure to accept their offer.
OTOH, if HD isn't paying, I wouldn't pay the extra money to use their high-priced stuff.