By the rule of all web technology sucks and is untrustworthy, they block 10.0.0.0 and 192.168.0.0, but inexplicably allow 172.16.0.0-172.31.255.255.

(Or at least, that was what someone else claimed last time this came up on HN)

My understanding is it allows them to check for things listening on loopback, given they appear to be checking for SSH tunnels.