The same reason why tor exit nodes are blocked/limited by services. You have the anonymity (if your opsec is good) and endless bandwith. So these will very likely be used for shady/illegal stuff. To do this with google or other providers would be not very smart, the majority of people will use these only for hiding your traffic from your isp, your local network or geo location spoofing and the percentage of suspicious activity from their IP ranges will be very small.