For the time being, since Binance has not touched the SBD, they may or may not be stealing. They are very unlikely to be the legal owner of it. It's a business decision in the sense that they aren't returning it or using it.

STEAL: take (another person's property) without permission or legal right and without intending to return it.

Take usually means you actively acquired it. However, it can mean obtain or acquire possession of something. Binance did not deliberately take it, so it was definitely not initially stealing and Binance had no intent to steal because they didn't touch it.

Binance never had permission from the owner to keep it. They may or may not have legal right to keep it. I don't think they have the legal right and they are not touching it which makes me think the answer is not obvious to them either. Besides, it's a sort of poor ethics now. If they spent it on anything without clear permission, it would be a reputation disaster.

It was sent to Binance on good faith that it could be traded. It was definitely not donated or abandoned (ie. send to null/steem.dao).

If Binance doesn't intend to return it, this closely resembles the definition of stealing. If Binance was made aware of it and said no, then they would need to prove they have right to possession.

You are right to say it isn't stealing because no legal judgement has been made, but a court could easily order it returned and refusal to do so would definitely be stealing.

So it's potential theft, accused of unlawful possession, pending legal action, etc.

I would say this sufficient to pursue legal action, it may not be thrown out of court. It probably isn't worth the effort, especially given the general status of cryptocurrency.

However, instilling more trust in Binance under these circumstances is not a good idea.