You are correct that the merchants have that choice.

Amazon's behavior is still anti-competitive. They are the big boys. The drive lots of volume, and have high fees. This policy makes it impractical for most vendors to support Amazon's competitors or compete with Amazon themselves. And it robs customers of a meaningful choice that would cut out an expensive middleman.

Without this policy, you might see lots of products on Amazon that are $19.99 with Amazon's 30% cut, and maybe $13.99 (30% less) on the vendor's own website. The consumer loses twice with Amazon's policy: once because they couldn't get the item at a cheaper price, and again because they didn't support any innovation or competition in the market, which would also lower prices.