Sorry, but you have not been told BS by TG. Your ticket did not include the THB 700 BKK airport fee as you were in transit (< 24 hours in BKK) but when you decided to leave the transit area, you became subject to the fee to re-enter the transit area. Nothing new there!
You may have been lucky in the past that the immigration officer (who is supposed to check that your BP is issued at BKK only) did not notice, but if you ask for a BP at the TG check-in at BKK (rather than to re-enter with a BP issued at the origin) TG will ask you to pay the supplemental airport tax. Obviously, if you stayed air side you would have not been asked to pay the airport fee.
As stated above this issue has been discussed many time in this thread.