First, because they fall foul of the alea law which says that anyone who uses a Nazi analogy knows nothing much else about the past.
Second, because Cruz thinks Chamberlain drew no line in the sand. Appeasement referred to the status quo. At the time, that meant something a whole lot less than the continent of Europe. Basically, Germany had invaded Czechoslovakia and Austria. When Hitler invaded Poland, France and Britain declared war under the mechanisms established in the treaty with AH. So actually France and Britain said they would stand against Hitler if he tried to take over Europe. They did so knowing they weren't prepared for a war but thinking French defences could buy them time.
Third, because McCain thinks America was the instrument of a change in policy towards Nazi Germany. Whereas in fact the US was a late entrant to a war that was ongoing. The US had tried very hard to remain isolated from the war with Nazi Germany. So for a few years while countries like Britain, Canada, NZ and Australia were fighting Hitler, the US supplied materiel but remained militarily unengaged. Not much of a pulpit from which to throw rocks or claim plaudits.
Fourth, wrong decade. Appeasement was so 1930s.
http://www.huffingtonpost.com/2013/09/25/john-mccain-ted-cruz_n_3989887.html
"If you go to the 1940s, Nazi Germany," Cruz said. "Look, we saw in Britain, Neville Chamberlain, who told the British people, 'Accept the Nazis. Yes, they'll dominate the continent of Europe but that's not our problem. Let's appease them. Why? Because it can't be done. We can't possibly stand against them.'"
"I resoundingly reject that allegation," McCain said. "That allegation, in my view, does a great disservice. A great disservice to those brave Americans and those who stood up and said, 'what's happening in Europe cannot stand.'"
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