Public Suffix List

I think you may be reading it wrong, both syntax and purpose. The section for '.th' reads:

  // th : http://en.wikipedia.org/wiki/.th
  // Submitted by registry <krit@thains.co.th> 2008-06-17
  th
  ac.th
  co.th
  go.th
  in.th
  mi.th
  net.th
  or.th

This means for any of the seven listed 2nd-level domains -- 'ac.th' ... 'or.th' -- the 3rd-level is considered the assigned-by-a-registrar level. Only if another 2nd-level domain, other than those seven, were encountered, would the list suggest it be considered its own level-of-assignment.

If there is such a 2nd-level domain inside '.th', that should be another public suffix, you should file a bug against the list with the Mozilla Project. (If it is policy of the '.th' registry that every 2nd-level domain with '.th' should be treated this was, an entry of...

  *‍.th

...should be added to the list.)

Also, don't be caught up on the 'public' part of the name -- it doesn't mean this is a guide to what domains are registerable by 'the public'. Many of the TLDs/2LDs/3LDs listed only allow subdomain-registration to specific entities, like governmental bodies. In my project, we call the subdomains that are one-segment-more than these 'public suffixes' "Assignment-Level Domains" (ALDs).

So if you're considering 'www.whitehouse.gov', and 'gov' is the longest suffix in the list, then 'whitehouse.gov' is the Assignment-Level Domain. If you're considering 'www.grad.mahidol.ac.th', and 'ac.th' is the longest suffix in the list, then 'mahidol.ac.th' is the Assignment-Level Domain. (It's one more than the best-matching suffix.)