Context is everything. In this part of the Republic, Socrates and friends are trying to work out an education plan for the guardians (I've seen the word "auxiliaries" used in some translations as well). These aren't meant to be the majority of the citizens in this ideal city, but rather a combination military and police force. The decision to portray the gods as good and without fault is made for the same reason the guardians will not be allowed to read poetry--it will be a corrupting influence upon them. In other words in this particular passage, whether the stories about gods' misdeeds are true or false is not at issue. The point is coming up with an educational plan that will create the ideal guardians for the city.

As for Plato's thoughts on the gods, though, the question isn't entirely straightforward. We have the obvious difficulty that aside from a few letters (many of whose authenticity is disputed) Plato only ever wrote in dialogue and moreover in those dialogues, Plato is rarely mentioned and he is never a main actor.

It's important to take note that when Plato deals with the gods, there is often a very thin line between the god and the idea he or she is meant to represent. The pantheon seem simply personified concepts. The party guests in the Symposium give speeches in praise of the god Eros but there is no real meaningful distinction as they do so between eros (erotic love) and Eros (the god of eros).

I'm not qualified to say whether this non distinction was unique to Plato or common in his culture, but within the scope of Plato's thought it certainly makes sense. Plato is constantly concerned with the truth beyond the physical, reality behind appearance. He speaks of the Forms (also translated Ideas), sort of celestial archetypes. Again in the Symposium the example is given of how you can look around and see beautiful people and in so doing you can begin to realize there is something they all share, that despite the differing presentations, they all possess beauty. So somewhere up there in the celestial reaches, there exists the Idea/Form of Beauty itself, sort of the archetypal or truest quality which all the beautiful things around us take after or point toward. And so too with everything from Good to Table-ness, that quality which makes a table a table.

The point is Plato sees the Forms and the pursuit of them as a decidedly good thing. In the example in the Symposium, the transition from beautiful people to regarding Beauty itself is a motion meant to continue, ending with you coming to perceive and understand Good itself. To Plato this is the end goal, to know Good itself (and this is why in the Republic they who know Good itself, that is the philosophers, are the kings).

All that to say, Plato was an idealist and a moralist but he doesn't seemed to have cared much about the gods as such, except where they intersected his ideas. So when he's talking about the education system of this military force in this imagined city, in describing the influences that must be kept away from the Guardians, he says the gods must be portrayed only in the positive, because it suits Plato's purpose there. Plato's goals are understanding how things really are and into that pursuit, for the most part, the gods don't seem to enter in.

Plato isn't alone in this. Philosophy is often described as beginning when various thinkers in the Greek world (the so-called pre-Socratics, though many were his contemporaries or even outlived him) began looking for answers to the big questions about life, the universe and everything, and doing so in distinctly non-religious terms. Rather than answering "Why should it be the case that _______?" with "Because Zeus wills it to be so," we get philosophers offering that everything is made of water, or everything is made of fire; change is an illusion or the world is in a state of constant flux. As part of that conversation, we should not be surprised that Plato leaves the gods mostly out of his thinking, puts his attention on other questions.

Uh, are you sure 'Plato is stating' things? Can you assume that the interlocutors in sources attributed to Plato are expressive of his opinions?