This is a general remark. When you have a policy whatever it is if you delete the policy you are not able to return to default behaviour this means that the policy (collect or not collect poll or not poll) is still active.
To remove the effect of a policy you have to set up another policy that states exactly the contrary!

BTW, when you remove a policy what happens? You should add a policy that resets the poll status of the OnmsSnmpInterface otherwise?

The solution is to add a default interval to snmp poller configuration file. All the interfaces that not matches in a package will be polled using this interval.

The use case is clear:

the provisioning system let via SnmpMatchPolicy rule specify if an interface is to be polled or not.

The snmp poller config file will let you to set the poll interval.
But what happens if the interface is to be polled but there is no package match in snmp interface poller config file?