In terms of the motivations, sure the guy is trying to stop people littering, but next thing you know, he's transitioned into selling ad space and is making his millions, all after free-riding on Instagram's goodwill in the name. I'm being facetious of course, but you don't know what will happen in the future. The context behind the use is also likely to have tempered Facebook's approach to the infringement. For all we know, they may have offered to help with the rebrand costs.
I don't know of many companies who adopt a completely over-zealous approach to trade mark enforcement. Do you know any that fall within the type you describe in your post?
Finally, I don't know who your lawyer is, but in the unlikely event the matter went to court, or to a trade mark authority's dispute resolution process, you would run an argument precisely in line with the one you describe. You would look to have the trade mark registration invalidated on the basis that it is descriptive and was therefore not suitable to be registered. I don't think you'd get very far but you'd make the argument all the same.