Were they any more naive than the scientists who built the bomb for the United States yet hoped it wouldn't be used in combat? It is easy to say, "oh, anyone who would have worked for Stalin had to be hopelessly out of touch" but there were people who, in retrospect, look just as out of touch on all sides. And we should not forget there are many ways of looking at this history (only one country, so far, as used nuclear weapons in battle, and they used them to exterminate cities — a view hardly appears peace-loving).

But I digress. You asked of the motivations. It differed by the spy in question. Fuchs was a hardcore Communist who had marched against the Nazis in Germany. He saw the Soviets as the allies in the fight and wanted to give hem every advantage. (And, again, they were Allies at the time.) He saw himself as working hard for the Americans, British, and Soviets. Everything I have read of him and by him suggests he believed Stalin was a great guy, that the Soviets were a good force in the world, etc. By the 1940s this was a less popular view than it was in, say, the 1920s-1930s, but it was still a possibility. The full "crimes" of Stalin were not well-known, and it was easy to point fingers at other nations for their own "crimes" (the US's approach to race relations was not exactly something to be proud of, nor was the UK's imperial colonies, and so on).

Another spy, Ted Hall, later framed his spying in terms of balance — that the world would be safer if the USA wasn't the only country with nuclear weapons — but that is probably retrospective. But it does indicate that these people who were working for both countries could see such things as being mutually beneficial to the world, as opposed to helping just an "enemy". They were building a bomb for the USA, first and foremost, but they also felt the Soviets ought to know about it.

Some of them were not especially ideological, though. Harry Gold, the courier for Fuchs and Greenglass, appears to have fallen into it almost socially — he was eager for friends and became "groomed" for his role. He doesn't seem to have had strong feelings about Stalin or the USSR; he did it for more personal, ego-related reasons.

So anyway. On the whole this generation of spies were more ideological than those who came later (who often did it for money or ego reasons), but I think calling them "naive" is perhaps not the best approach. Everyone is naive in retrospect. I also don't think the idea of deterrence is a "bizarre idea" — it is only bizarre if you truly believe that the USA would never have its own aggressive or imperialistic ambitions. Looking at the actual history shows this is clearly not the case — the US has also overthrown democratically-elected governments, started wars, assassinated leaders, kept colonies, etc., when it thought it was in its own interests.