(UNEDITED) There are many reasons for people requiring to scratch their 'rear-ends' in public. Uncomfortable underpants, nylon knickers, tangas, fleas, and the most notable cause is; inappropriate use of bog paper.
Bog paper, if not used properly, and missing the point, can cause a severe itching of the rear-end. Proof of this is when babies poo in their diapers leaving a rather itchy, red patch in the area, requiring moms, or dads, to apply zinc cremes. However, this is applied either at home, or when babies are travelling, in separate bog booths for moms and babies, if they're lucky.
India, and many other Asian nations, regard scratching one's posterior in public as a severe crime because they never use bog paper. Instead people, not babies, flush their rear-ends with running water, a very hygienic way, by the way, and Indian toilets are never blocked, only those in slums where there is no running water, just filthy, contaminated puddles.
Resulting in the fact, that if adults, with running water facilities in their homes, not slums, are caught on buses, trains, or walking down streets scratching their butts, they will receive a fine. If they become repeat offenders, this could lead to a prison sentence.
Health authorities in India and other Asian nations regard scratching ones butt in public as rude and unacceptable behavior because they observe western nations using masses of bog paper, blocking up sewers and, many times, missing the point, leaving stains on underwear which require heavy chemical detergents to remove them. In India, bog paper does not exist, so authorities say:
"If people cannot clean their bums correctly with running water, they are themselves to blame, and if they have itchy rear-ends, never scratch in public, it is offensive for others who do the job correctly!"
Rich southern Europeans and South Americans have bidets to do the job, and are allowed to scratch their butts in public because bum-scratching is regarded as sexy, for some!