Stealing is wrong. But the bigger picture here is that God's plan overrode that of man. Solomon set up the Queen of Sheba because he wanted to sleep with her. His excuse of course was that he wanted to spread the Davidic bloodline around the world. But he could not see the fact that in this regard, God had other ideas. That is why despite having women brought in from around the world, he only had 3 sons- including Mennelik.

God had chosen the Queen because she had immediately converted to Solomon's religion as soon as she had received instruction in it, unlike the Israelites who were always straying. This was all the more remarkable in that she was from another country, from a lineage and heritage much older and more exalted than Solomon's. Solomon's religion was newly established, he was building the temple etc.

God wanted the Ark removed without a bloodshed. The Israelites would have not let Mennelik take the Ark, they were yet to recognise that the Glory had left them. So, "stealing" it was the only way to do it.

My own personal history asserts that my dynasty established itself through the aid of a daughter who seduced the local ruler and then did a Delilah on him. It was wrong, but my fathers had no choice and when he was defeated he admitted that he had lost the land and they could have it. He did leave a chilling reminder, there is a place that has these vibrations that can render any one of my clan insane or have epilepsy or some other nervous disorder.

The Kebra Naghast tells us that Israel's loss was Ethiopia's gain. Solomon lost everything when he "scored" with a visiting Queen. But, God had made a promise to his fathers, and this was passed on to the son of the Queen.