They're not being dumped. Putting them on YT lets WB make passive money while maintaining control of their rights with little effort on their part. If WB makes a better deal down the road, they can hide or delete the movies from YT.
This also makes some of the movies more valuable by revealing hidden demand. WB will see their YT stats for their films and see where future investments or licensing deals may pay off. A streaming company is disincentivized to tell the movie owner how the film is doing.