I don't know if it is infill or not but it's more Infill than Top/Bottom layer. I cannot see any reason this would make sense as a feature. All it does is take away the option to print this part at a different speed. If a person wants the 100% infill to print at the same speed as top/bottom layer he can set it that way, this way the choice is taken away.
Anyway I normally don't print at 100% which is why I am just noticing this now, Bug or Feature it's annoying for sure.