There are a number of reasons that it is relevant to mention the JW connection. Here are a few that come to mind.
Castro's mother has likely been a JW for quite a while, a lady in her mid 70's likely who became a JW as a young mother. I'm guessing, but we have all known of many young mothers like that. If so, most likely this Castro and his brothers grew up attending a KH. Surely someone on this forum from Cleveland knows this lady and can give some details. A Spanish congregation maybe? It is astonishing that no one had a clue he had three captives for all those years. Surely someone's spidey sense tingled sometime. If Castro ever attended KH, then various elders and JWs would have known him personally. Scary.
If Castro has not been working as a bus driver since he was fired, where has he been getting money? Hmmm. You can't maintain a home, pay bills, buy McDonald's food, without income.
How much you want to bet there are Watchtowers and JW literature in that house?
As someone already mentioned, the Jaycee Dugard story had a JW connection too. Simply a coincidence?
As this story unravels, I think we will be both horrified and astounded.