They are for profit business. Even grocery chains have behavioral psychologists. Valve doesn't run the gambling sites, they even tried to stop it multiple times. [1][2]

Valve doesn't pocket anything from the direct trade. Their cut is the same regardless the selling price of an item, except when traded through steam marketplace.

The only benefit valve stands to gain from this is the free marketing. Their business in this sense is not much different from trading cards, except the goods are digital.

[1] https://www.hltv.org/news/43421/valve-bans-skins-gambling-ca... [2] https://www.polygon.com/2017/1/30/14440958/team-fortress-2-g...