It costs nothing for anyone -- ARRL Member or not -- to submit QSOs to LoTW, and monitor their confirmation via LoTW. Some DXers are perfectly content with tracking their progress in this manner.
It costs money to submit confirmations to the ARRL to obtain award credits for DXCC, VUCC, WAC, and WAS awards.
This is a classic Freemium business model: by making the submission and monitoring free, a large number of hams around the world are motivated to participate, even if they themselves are not pursuing ARRL awards - or would be if LoTW were easier to use and localized to their native language.
This model is particularly effective because LoTW eliminates the costs of postage and time required to wrangle QSL cards, making it more palatable to pay for ARRL award credits.
From a revenue generation perspective, it would help if the ARRL actively promoted its awards.
73,
Dave, AA6YQ