I fail to understand the following action from LH.

Flight LH-metal VIE-FRA. I choose seat 5C (economy ticket) at OLCI. I arrived at the gate, presented my boarding pass and received a nice paper stating that I am now seating in seat 8C.
I understand the possible need for LH to move the Business-economy curtain, when number of C-class passengers is higher than planned (happened many times, thanks!).
However, this was not the case. There were only 4 pax in Business (rows 1, 2 & 3). The remaining seats and rows were totally empty.
Thus I sat in the 5C seat, presented my BP when requested by FA to check the presence of 5-pax instead of 4, as written in their manifest. After 5 minutes, FA came and informed me that they will exceptionally leave me in 5C, even if it is now a business seat, but will not have any meal available.

why do LH change the curtain position when the business class is empty? It takes 5 seconds to move the curtain back and forth if needed for the next flight...
The answer I got from FA was that LH needs to show all customers that their business-class section is important, thus decided to enlarge it.
Fail to understand their logic.