The World MuayThai Council Rules (WMC, 1995)
section 18 (the section that outlines foul and strikes violating the rules)
rule 18.2 wrestling, back or arm locks or any similar judo or wrestling hold are illegal.

Board of Boxing Sport Rules (Sport Authority of Thailand, 2002)
rule 16.2 states that, throwing, back breaking, locking opponents arms, using Judo and wrestling techniquesare all violations of the rules.

The Amateur MuayThai Association of Thailand, the Federation of Amateur MuayThai of Asia, and the International Federation of MuayThai Amateurs rules for Amateur MuayThai competition include rule 17 b (2) a throw on an opponent, a back breaking manoeuvre, an intentional fall over, a judo or wrestling throw to an opponent are fouls.

The following judo and wrestling moves are fouls (Board of Boxing Sport, 2002):

a) A boxer wraps his leg around the leg of their opponent from the inside or the outside and forces their opponent to the floor.
b) A boxer locks an opponents neck and executes a hip to shoulder or hip throw
c) A boxer grabs their opponents hip in a waist lock and throws using a hip throw
d) A boxer grabs an opponents arm, turns and uses their calf and back of their thigh (hamstring muscle group) to sweep an opponent’s legs from under them.
e) A boxer grabs an opponent and falls backward to throw their opponent
f) A boxer attempts a rugby style tackle on the legs and waist of an opponent
g) A boxer grabs their opponents waist from behind, then, placing a leg between their opponent’s legs, pulls their opponent backwards over their leg and hip
h) A boxer grabs their opponent from below the waist, lifting and throwing
i) A boxer catches their opponents leg and uses their other arm to push them off balance while taking more than two steps
j) A boxer catches their opponents leg and uses their calf muscle to trip/sweep an opponent off their feet
k) A boxer ankle trips their opponent
l) A boxer leg sweeps their opponent using the calf or inside of their foot
m) A boxer traps their opponents arms in a waist lock, lifts their opponent and throws
n) A boxer grabs an opponent from behind, lifts and throws
o) A boxer grabs around an opponents lower spine and hyper-extends their back in a back breaking move

Go ahead, cite something that backs up YOUR argument....