As stated by Nick Art, allowing *G in LH group lounges is a profitable business. LH has an incentive to attract *G in lounges and even more in SEN lounges, as I understand that LH managed to negotiate with *G partners more revenue from a *G passenger entering a SEN than a C lounge.
The fact that certain airlines offer *G too easily is not an issue as far as LH is concerned, as such airlines bear the cost of their passengers entering LH group lounges.