I don't know much about Subway sandwiches other than Jared supposedly lost a ton of weight eating them.

It's been a while since I have been in a Subway shop. I consider Subway "3AM ... no other choice...drunk food". Around here, during the day, there is a plethora of really good deli's and Italian deli's where you can get far superior homemade hero sandwiches. On real bread too.

I would guess they ask to toast the tuna to make a psuedo tuna melt sandwich. A tuna melt can be a very good sandwich if it is made right. I wouldn't think Subway is a place that would make a good tuna melt.