Yes its technically illegal to ‘steal’ their wireless connection. Its also a breach of your ISP contract (in most cases) to even broadcast it though.
But I have my own thoughts on this. In some cases the wifi signal from your neighbors can be broadcast into your home. If someone is broadcasting an unsecured wireless signal INTO MY HOME. Damn right I’m stealing that shit. Every AP has the ability to be secured to at least a basic level. IMO, it’s up to the owner of that AP to secure it.