Why would you be surprised that bookstores have books about
opposing beliefs? Bookstores stock books that people want to buy.
They don't really care about the content of the books they
carry.
The government does regulate what books can be sold. For example,
you can't buy the Jehovah Witness' version of the Bible. Books
about politics in Singapore are also carefully regulated.
I think some bookstores may give Lee Kuan Yew's book more prominent
placing on shelves, but in general, a bookstore is just a retail
business and retail businesses stock what sells.