Hi Kirk,
Muay Thai punches and hand guard were adaptation of western boxing punching technique. It had been modified to suit their kicks, knees and elbows combo. Its also adapted to suit Muay Thai way of judging. It developed into more power based as judges seldom score on punches unless it staggered or knocked down the opponent. The flashy footwork in Boxing also been discarded as it was a weakness to leg kick which could swept the opponent off the floor easily. The bob, weave and dart were modified to limited use as they open up the lower half of the body for attck.
There is nothing wrong to learn and adapt from others. It happens all the time in sports, science and technologies. When Muay Thai became a boomed industry in the 30s, all that the fighters wanted was to win and increase their rating which would result in higher earning. No body really cared where the techniques originated as long as it helped them to win. If any new effective technique were to developed today, be it American, Indian or Eskimo, Muay Thai fighters would adapt it for their own benefit.
After Muay Thai developed into a professional sport, there is one noticeable change which some of us in this discussion can learn from. The different traditional styles among Muay Thai became blurred or disappeared as every style tried to learn from each other in order to find the best way to win in fights. Today, if you were going round Thailand, thumping your chest and saying " I am from Kxxxxx style, we are the best style in KNOCK DOWN and KNOCK OUT fight"( sound familier? Mark Hampton of NZ), you probably will receive a puzzled and incomprehensible look.
By the same token, to be fair, one should not penalise Sanda for fighting like Muay Thai. All sports learn from one another be it swimming or soccer(I got it right, Collin H and NO arguement please!) or others. If we look further back, all fighting styles have learnt from from one another in the course of historical interaction be it Karate(China) , Tae Kwan Do(Japan), Judo(China thru, Jujitsu), Chinese Martial Arts (India) or Muay Thai (India).