Why does Shimano mood a spacer to hg road casette?

I wanted to change the casette on my commuter bike. Without giving it much thought, I figured HG is HG and so a road casette would fit my mtb. However, when I wanted to put the 11s casette on, the smallest cog couldn’t grip the splines on the freehub body. I noticed the spider had a spacer casted in as part of the design. A few minutes with a file fixed this and now the 11s 105 casette works perfectly on my 11 speed mtb hub with xt shifter and deraillieur.

Question: why does Shimano do this? Are they trying to segment the market more? Is casting the spacer cheaper than adding a spacer to the packages? Am I banned from this sub for doing this?
I need answers people!

by LaurensVanR