With all due respect you obviously have little experience with this. I know many airlines that can not be ticketed on the same ticket as Delta and they are not LCC's. I know three people who recently booked a trip to India with 3 tickets. Combination of price and interline ticketing issues.
Considering you can book both Jet and Air India on a ticket with DL, I suspect your friends' issues were either price-related or LCC-related. I won't say that it is obvious you don't have much experience with this, but there is at least some circumstantial evidence to that effect.