You use self signed certificates all the time with SSH. If you haven't seen an SSH key before, you don't fall back to telnet.
If you stop to think about it, is incredible how much effort they put into forcing you to use the latest browser, and not trust self signed certificates. It is far easier to root your device and patch your kernel than it is to use an older browser.
Yes, it is a highly effective but clumsy heuristic to detect abuse. But I am convinced that they may have other incentives as well.