Are physics and natural sciences considered liberal arts?
There is little consensus, I think. But mostly it is a matter of convenience and organization at colleges and universities. I studied mathematics. I earned both a BA and an MA at different places, one of them an R1 university. Just by asking, the degrees could have been designated BS and MS. Other places the math department is in the School of Engineering. But that is just an organizational convenience, though it might let some departments share faculty more easily when it comes to budgets and such. Some places CS is part of the math department and is a "liberal art". Other places it is part of an engineering discipline. Still other places it is different from both. 

But in the US, at a liberal arts college, the student is expected to gain a broad and well rounded education. It will include both literature and science, even though it might focus somewhat on a single area. 

But, your question, itself, details that there is no consensus on this.