They use one license on multiple ts3.
The side port of this thread on this thread:

Having two servers with only difference being ports doesn’t mean that someone is violating license, as any license that is not free 32 slot one or paid 64 slot one has two virtual servers. This means that one server owner can have two distinct servers running at the same time, eg. one for normal use and second for testing.

If you think that server owners are indeed violating license, please create a ticket here.

Just to clarify, that posted link automatically opens up email client installed on your device, be it PC or smartphone. In case you do not have any, email is [email protected]