I don't think they stole it. Back in th early days of the Japanese auto industry, other manufacturers shared designs with them, I am sure never expecting them to be as successful as they are now. I've been told that an early straight six from Toyota will freely accept parts from the GM motor it was based on. Now, I have no idea how similar the Porsche and Subaru engines were.

 

Shared maybe, but the Japanese back then had the reputation that the Chinese have now. And that is, a lot of the things they did were considered cheap knockoffs. Maybe some of these "shared designs" were sold by Americans illegally to other companies. Either way, several other manufacturers have used the boxer design, just not as extensively.