The musician doesn't write their own riders, they just say some things they want included, and everything mentioned in Katy's is pretty standard. Clean food for her and her staff (Oh no! How crazy!), make sure that the furnishing SHE PROVIDES is placed in her dressing room, the venues staff shouldn't ask her for an autograph because that's unprofessional and would distract her from preparing for the show, etc.
Imagine she went to a venue and every couple minutes someone from the burger van came along and knocked on her door to ask for a photograph? Ridiculous. (It's probably written into the staff contract anyway, but it's best for her to restate it regardless).
The show would be a train wreck if the tour rider didn't state exactly what is required of the venue to do to prepare for the show. It contains all the technical and logistical details of running a smooth show from the moment of Katy's arrival to her departure.
Katy was asked about this when her California Dreams Tour rider leaked and it stated that fresh flowers were to be provided, but absolutely NO Carnations, and I'm pretty sure she laughed it off and said something like "You'll have to ask my sister what her problem with Carnations is, I like them, she's the one who wrote the rider".