Daily Puzzle Issue

How can I say that empasent is available?

By process of elimination. There are no other tactical ideas in the position. That said, the Daily Puzzle is less flexible; if that had come up  as a regular puzzle, black's previous move would have been show. Daily puzzles don't have that luxury.

I know. So, why don't add it up?

It is a general rule of chess puzzles that all privileges that are not logically excluded from the displayed position (castling, en-passant...) are available (unless mentioned otherwise).