That's not altogether accurate....
The assistant referee has no "authority to call" anything. He/she can certainly signal locked hands, illegal holds, unnecessary roughness, etc., but the head official can overrule his call and does not have to abide by his signal. Normally, however, the head official would not overrule the assistant if he/she did not see what the assistant said that they saw (locked hands, for instance).
I believe in the matter of the "slam", the assistant official, from his angle, saw that the defensive wrestler's arms were tied up to the point that he could not extend them and therefore protect himself, whereas the head official could not see that from his angle. It may (emphasis on may) have been not so much the force in which the wrestler was taken to the mat, but the fact that his arms were tied up and therefore he landed awkwardly and was injured. The offensive wrestler is responsible to insure that the defensive wrestler is returned to the mat safely. Unfortunately, when the move occured with 30+ seconds to go, it was not determined that the defensive wrestler was injured for nearly 7 seconds. At that point the officials discussed what occurred and the call was made. Had the defensive wrestler been able to continue, the clock would have been reset to 30 seconds, as that's when the act occurred.
I agree, having mat judges on the sides to make calls and/or overrule the mat official might be a good option, especially at the state level of competition. Suggestions such as these can be made to the National Federation at www.nfhs.org , and your's is a good one