Freedom of speech means that you can't be stopped from saying or expressing ideas, even if they're offensive to a lot of people (with certain exceptions; shouting "fire" in a crowded theater is not considered protected speech.) On the other hand,
nobody is required to provide you a platform for your speech. No publishing company is required to print your manuscript, no radio station is required to give you free air-time, no bookstore is required to sell your book, no search engine is required to show you in its search results.
Amazon, for instance, doesn't sell some books on
go that I'd like to buy. That doesn't mean they're censoring go books. It just means that the subject is esoteric and that the books sell in volumes too small to make it worthwhile.
So it wouldn't strictly be
censorship for Amazon to not carry this book, or holocaust denial books. It
would be censorship if the government, say, shut down the publisher or anyone who sold the book.
On the other hand, I understand the hesitance to block the sale of something based on its content. For example, there are lots of books they carry that could be construed as instruction manuals on how to commit worse crimes, such as murder (mystery books, the anarchist's cookbook, chemistry books that have instructions on how to prepare poisons, drugs or explosives, etc.) Creating a bright-line policy would be difficult.