[zmgsabst]

Even in strict logic, “is” can denote membership, as in, “squares are rectangles” does not entail “rectangles are squares”.

[1718627440]

Because it is "(all)squares are (some)rectangles", so the reverse "(some)rectangles are (all)squares" is also true.

The difference is how you speak about a group term, not the meaning of the word 'to be'.