Good question. The melting point on the low end of PVC plumbing overlaps temperature of boiling water. 99% of the time, the contact with the boiling water is so brief it won't be a problem. But, if that water gets stopped by a clog somewhere down the line, the prolonged exposure to the hot water can cause the plumbing to become warped. Most (but not all) PVC P traps are formulated with a PVC version that can handle temperatures above boiling water, but I have come across a couple that didn't survive over the years, with the first one happening to me which led me down the rabbit hole to find out why it's not a good idea to just pour boiling water down the drain.