It is true that the labour laws do not distinguish between a PR and a non-immigrant however in 2018 violating those laws as an employee was largely decriminalised. Whereas previously the maximum penalty was 100K fine and 5 years imprisonment it is now a maximum of 50K fine and there is no imprisonment.
https://home.kpmg/th/en/home/insights/2018/08/tax-news-flash-issue-44.html
https://www.tilleke.com/wp-content/uploads/2018/05/IC-May-2018-Relaxation-of-Work-Permit-Requirements-in-Thailand.pdf
On the other hand section 37 (1), Chapter 4 of the 1979 Immigration Act says the following;
Section 37 : An alien having received a temporary entry permit into the Kingdom
must comply with the following :
1. Shall not engage in the occupation or temporary or employment
unless authorized by the Director General. or competent official deputized by the
Director General . If , in any case , there is a law concerning alien employment
provided hereafter , the granting of work privileges must comply with the law
concerned.
The maximum penalty listed in Section 75, Chapter 7 is;
Section 75 : Any alien, who fails to comply with the provisions of Section 37(1)
shall be punished with imprisonment not exceeding 1 year or a fine not exceeding
10,000Baht or both.
Chapter 4 of the Immigration Act covers aliens granted temporary permission to remain (non-immigrants). Permanent residence is covered in Chapter 5 and makes no reference to employment at all.
Therefore I believe that a non-immigrant caught working without a work permit violates both the labour and immigration laws so can potentially be fined and sent to jail whereas a PR can only be fined.