This, of course, is the message in "Bridge on the River Kwai," where the tension is between resisting the Japanese on the one hand and on the other showing them that "they ain't no British" when it comes to work or engineering. And what David Lean seems to have missed is that in fact there MAY HAVE BEEN a greater morale victory in actually building the bridge than in resisting the Japanese. Jesus said as much ("Overcome evil with good.") Now, I'm no Gandhi, but there may be instances where you gain a greater victory by either building something HE cannot build (the bridge) or destroying something HE cannot build ("The Fountainhead"). These are all deep metaphysical questions that I sure don't have answers to, but heck, I know enough to ask questions.
Most of the professionals were barbarians, probably prisoners of war who somehow escaped summary execution on the battlefield per ancient rules of engagement. Many of these were already professional warriors from warrior cultures. They had been raised to fight and expected, sooner of later, to die in battle. The arena was not the battleground they had sought, but for many of them, fighting was preferable to slavery. So they fought, and a few survived and prospered. Some even continued to fight after they had gained their freedom and were free to walk away.
You referenced Spartacus. We need to remember that Spartacus was not Kirk Douglas. Spartacus and his army could have escaped had they wanted to. They had defeated the initial Roman attempts to catch them and fought their way to northern Italy and the Alpine passes. They were free to go. But they turned back to loot and pillage. It turns out the gladiators were barbarian pillagers to the end.
Of course, there were also plenty of suicides among people condemned to the arena. I've never seen an estimate about how common that might have been. But in the context of the HG/CF movies, the motivational structure of the careers is certainly not unprecedented.