Its to do with it being cooked all the way through and not bloody or pink. They advise it well cooked (read tough as old boot leather and blergh to eat therefore) and served fresh.
TBH beef is not like chicken in terms of needing to cook all the way through. Any bacteria are on the outside so cooking it for couple of minutes either side will sort that out.
I adore steak blue (flash cooked either side and served) and very rare and could not bear the thought of a well done chewy steak so never ate it. I had one medium one around 9 weeks PG and even that was
I'd say it was personal choice end of the day. I can't imagine Subway serve decent quality steak anyways. Cheap and nasty for their fillings methinks. Yes I am that fussy when it comes to steak