This would imply that people are capable only of doing one thing at a time and, potentially, that they're only capable of doing one thing full stop. I would honestly be amazed if a single person, anywhere[1], looked at this change and thought "yep, I don't have to think about slavery now".
> think of themselves as somewhat environmentally responsible.
Well, I guess they're more environmentally responsible than if they weren't recycling plastic. It's better to do something, however small, than nothing, surely? (And I doubt their environmental footprint even registers in the grand scheme of things - it's industry we need to shame, not individuals for the moment.)
[1] Who wasn't already heavily invested in ignoring the repercussions of slavery, etc., I suppose.