Probably not an oversight, the same thing can be done from any laptop by manually configuring any desired (or erroneous) IP and connecting it to a network. It is up to the ISP or provider to prevent such packets from going out to the Internet. The extended ping command is for privileged users in enable mode who presumably know what they are doing. The Adtran firewall when enabled has reverse port forwarding protection so a user behind the device will by default not be able to source packets from outside the network to which the device is connected.
This capability is very useful as a diagnostic tool in VPN and MPLS scenarios, as well as testing NAT traversal and firewalls.
Search "BCP38" for more details on the issue.
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