Why is Yasuo allowed to dash out of J4's ult?

For it's a dash, simple as that, every other champion in the game can do it who has one.

Why is he allowed to nullify every champions ultimate in the entire game?

Because everyone is allowed to dash through J4's wall.

Uh, it's not an impassable wall. Anyone with a dash is free to go through it. This includes Yasuo, whose E can also dash through actual walls.

In fact, J4's ultimate is even less than a normal wall. Vayne can tumble through it (but not through other walls), and Vladimir used to be able to pool through it.

all terrain is impassible in the rift. yet there are many abilities that go through walls. can jump out can ult out can ult out of j4 ult can bandage toss a minion to get out can use her jumpback+net to get out can W out of j4 E, tumble, can get out of jarv ult can use her E spider form to dodge most of j4 ult can E out of j4 ult

i can continue if you want.