I'd argue the opposite- it's like saying that water is legal but it's illegal to drown someone. Nothing in the field of machine learning is in itself illegal,

I'd argue the opposite- it's like saying that water is legal but it's illegal to drown someone. Nothing in the field of machine learning is in itself illegal, as opposed to things like hard drugs and heavy weaponry, which are in and of themselves illegal for private citizens, regardless of what they're doing with them. While we're at it, whenever anyone talks about machine learning they're not talking about a technique of mathematics, they're talking about a technique in software development (albeit that has some mathematical roots.) It's not popular because we're all passionate about discovering mathematical truths, it's popular because we can use it to make computers do cool things. :)