A quadrilateral is a rectangle if it has four right angles. Squares do have four right angles, so they are rectangle.
Squares are particular case of rectangles, in that they also have four sides with equal length, but that's the point: they have something <em>more</em> than the generic rectangle, not something less.
So, squares meet the condition to be a rectangle, and have an extra property that makes them "special" among rectangles.
