lens - Why are so many kit-lenses parfocal if it's an expensive feature?

According to this answer, parfocal lenses (lenses that can zoom in/out without losing focus) are relatively rare and more expensive.

However, the original poster's kit-lens is parfocal. Additionally, the kit-lens that came with my Sony a390 and the kit lens with my brother's Nikon D3200 (both entry-level DSLR's, with presumably cheapo kit-lenses) are parfocal as well.

Does anyone know why, if this is a feature primarily of expensive lenses, so many cheapo kit-lenses seem to support it? What features of the lens cause it to be parfocal or not?