They detect lower than usual TTL of packets coming from your mobile device when it routes packets during tethering. They might also set TTL to 1 to packets going from internet to your mobile, so they get dropped instead of being forwarded further. You would need to plug the SIM card into your own modem+router combo with TTL modification rules in place to avoid detection. But then they might just block your modem by checking IMEI…
It is unintended, a bug that is already known and will possibly disappear when support for websockets is removed (i.e. only plain HTTP traffic from you web browser to the server will remain).