Why is that sabbatical gap is so much questioned, if someone is on sabbatical then how does that person need to justify the gap?

I guess that leave includes in your working period. And its just like annual leave after 7 years of working as university teacher/ lecturer its your right to take that leave with pay. So i guess you dont need to explain that gap as its included in your service. 

that's true as a practice but justifying to get a job is very difficult