Well... it's not normal is it? As a child did you ever kick things with your shin? No, you kicked with your foot.
I'll take into account he is an MMA fighter and this may be more natural to him, but aside from strengthening your shins i cannot fathom why anyone would want to kick something with their shin... think about volleying a footy, you can generate loads more power if you connect with the dorsal (top of your foot), if you miskick and hit your shin you don't have as much power in your volley.
A couple things wrong with this.
1) Basic instinct does not equate to best possible method all the time. Kicking with your foot is instinct, but kicking something hard (AKA someones head or someones shin) will result in all sorts of broken bones when using that method. The shin is altogether harder and a more concentrated point of contact. There is far less surface area on your shin than your foot which means the force isn't spread over a larger area and dissipated. Also, your shin is a single mass, as opposed to many, many bones making it soft and malleable. Think of it as hitting something with a steel pipe vs a sock fully of marbles. Yes, those marbles will hurt like a bitch, but they won't KTFO you like the pipe.
2) A soccer ball is NOT a skull. It's much, much softer. Yes, you get more speed at the end of your foot (It's the end of a lever), but if you hit something hard with that much force with your foot and it's many tiny, soft bones, it's going to shatter.