All right. Here's some explanation.

Apparently on Ponibooru, people started posting rule 34 pics of ponies, including the Faust pony. Users censored the pics by inserting a Subway sandwich. The sandwich was used often for censoring Faust rule 34 pics, so users decided that every time safe and not-safe Faust pics were uploaded on Ponibooru, they would insert a Subway sandwich for the lulz.

I have not seen Faust rule 34 pics yet, but I'm sure they are on Ponibooru. I DO NOT RECOMMEND LOOKING AT THEM if you despise rule 34 like me.