You did not specify the country of your passport, so it might be different for non-US passports.
For the USA passport, according to Department of State ("Will someone be able to read or access the information on the chip without my knowledge (also known as skimming or eavesdropping)?" question), closing it is enough to prevent reading its data:
We use an embedded metallic element in our passports. One of the
simplest measures for preventing unauthorized reading of e-passports
is to add RF blocking material to the cover of an e-passport. A
passport has to be physically opened before it can be read. It is a
simple and effective method for reducing the opportunity for
unauthorized reading of the passport.
So at least for the US passports, that's a way for the companies to sell more stuff.
You can also verify it yourself if you have an Android phone with NFC reader (most modern Samsung phones do). Just install something like NFC Tag, and try to read your passport.