> Both to blame

IANAL but I think the US law approach is to rely on chaining, so the #1 blame is on Sony until Sony proves it isn't.

1. Consumers who were damaged sue Sony for damages.

2. If Sony loses, Sony sues Studio Canal for damages.

3. If Studio Canal loses... ?

Correct. In theory, Sony should have warranted that they have the rights to sell the thing the way they sold it. If they didn't have the rights to sell a movie perpetually, then that's on them.