Memes

But why do people think there is some sort of contradiction?

There are different definitions of "free will", but the common one is purely idealist in a sense that our thoughts aren't guided by our material conditions. It's also often a religious position that god gave humans a soul and therefore only we have "free will". If you drill down to the fundamentals of that position you reach a position that says our thoughts don't (need to) obey the laws of physics and similar universal laws. It's a position of idealist dualism that states our "mind" is not material and is separated from the material reality we exist in. It very often follows that material reality itself doesn't really exist, except in our "mind" and then you reach a purely solipsistic position. That's why there is a contradiction. If the definition you're using for "free will" is basically just our material will, our thoughts, then the contradiction disappears, but I wouldn't call that "free will", as it will cause more confusion due to the definitions.

Here's Lenin from 'Materialism and Empirio-criticism':

The materialist elimination of the “dualism of mind and body” (i.e., materialist monism) consists in the assertion that the mind does not exist independently of the body, that mind is secondary, a function of the brain, a reflection of the external world. The idealist elimination of the “dualism of mind and body” (i.e., idealist monism) consists in the assertion that mind is not a function of the body, that, consequently, mind is primary, that the “environment” and the “self” exist only in an inseparable connection of one and the same “complexes of elements.” Apart from these two diametrically opposed methods of eliminating “the dualism of mind and body,” there can be no third method, unless it be eclecticism, which is a senseless jumble of materialism and idealism.

Note that the "complexes of elements" used here basically mean our sensations of reality, but it's a confusing term introduced by empirio-criticists to "smuggle in" idealism into materialist philosophy which is what Lenin is critiquing.