Well, depends on their budget and traffic I guess. A lot of images seem to be uploaded there, lots of memes for that matter so they could regularly be linked to all over the place - that could potentially be a massive drain on bandwidth.

They may also have only done this with SSL because it's more expensive than general HTTP traffic, so they might allow hotlinking as long it's not over HTTPS. Whatever is happening there I don't think it's broke, it's probably deliberate.

I just can't confirm if it's specifically the referrer without testing it, if it does it no matter what website is linking to them then it's about hotlinking in and of itself.