One of those cultural things, such as giving out a single tiny paper napkin when you buy an ice cream (rather than letting you take as many as you like) or the folks who switch off power to their apartment when they go out for the day.
Instead of an abundance mentality (let them browse the books: they might buy more than one, and it least are likely to be more satisfied) it's a zero sum (they might look into the book, read what they want, and then buy nothing).
I buy a lot of used books and of course this shrinkwrap nonsense doesn't apply there.
I don't see this in English speaking countries, India, Japan, or China; I do see it common in France and Germany. I don't buy books in other countries so can't say.