I find strange articles from a lot of the random shite I read on the internet. Mostly I steer away from "normal" news and head straight for the bizarre. It is more interesting.
This happened in 1972 when Led Zepplin was really killing it as far as their career was concerned. They were on their way to perform a tour of Australia and just like is still true today, they figured they would do a stop at the most convenient large Asian city on the way there from Europe (all of us in the rest of South East Asia normally just get bypassed, this is still true today.)
Anyway, the reason why Led Zepplin was denied entry is more silly than anything else. It wasn't because of drugs or criminal records or not qualifying for work visas or anything like that. It was because it was almost completely forbidden for men to have long hair in Singapore during that time.
Obviously, this applied to a great many more bands in the 70's than just Led Zepplin. Nearly all rock groups in the early 70's had men with long hair. LZ was just the first to find out about it. The Bee Gees were also denied but they got there 2nd. The BeeGees were actually right on the cusp of what is considered "long" hair in Spore.
The craziest thing about the turning away of Led Zepplin was that the show was already booked and it wasn't until the band was already in Singapore, with their private jet on the runway, that they were told they would have to get haircuts or not come in. They weren't even allowed off the plane.
Needless to say, Singapore is slightly less authoritarian today.