It's not technically a bad call because they have A5o. A6s vs 44, you have ~55% equity when all in if you don't have any of the A6 suits in your hand.
Assuming it's just the blinds + the 15 BB jam, there is 16.5 BBs in the pot and you have to call 15 BBs. If we plug this into an EV formula we get:
EV = [16.5 BBs x .55] - [15 BBs x .45]
EV = [9.1BBs] - [6.75 BBs]
EV = +2.35 BBs
However, even if it is a +EV situation for us, we still have consider stack preservation in MTTs. So, even if are averaging 2.35 BBs in the long run, it might not be worth risking 50% of your stack in this situation.
The problem is that is you need to calculate your equity vs your opponents shoving range and not just a specific hand. Your opponent would have to have a shove range of 45% to make 44 a slight favorite of 51/49. Most players aren't shoving this wide for 15 BBs. Even if your opponent's shove range is 93%, you only have ~57% equity to win. Going back to stack preservation, is 57% equity enough to risk 50% of your stack?
So, it's not a bad call because it's A5o vs 44. It's a bad call because you cannot beat your opponent's preflop shove range with 44.