Yes its a regulation, its mentioned in the Nationality that the Candidate "must have an occupation"
Summary here -
Under a 2008 amendment, males married to Thai females are now exempted from the requirement to have permanent residence and knowledge of Thai but not from the requirement to have an occupation in Thailand. In order to qualify for the exemption, men with Thai wives must have been legally married for 3 years (1 year if they have a child together).
In addition to the qualifications specified in the Act, current ministerial guidelines require the following. You must show evidence that you have a job in Thailand paying a salary of at least 80,000 baht a month (40,000 if married to a Thai) along with tax receipts going back 3 complete calendar years. You need to present a valid work permit and originals or copies of expired work permits, if you have changed jobs within the period. You should be in a house registration book (blue TR14, if a permanent resident, otherwise a yellow TR13).