To understand what was going on with licensed and unlicensed ports of popular arcade games in the 1980s, you have to understand two critical factors.
- The video gaming culture of the time, and the preeminence of coin-op arcade games.
- The role of trademarks and legal trademark protections, which was the more crucial law pertaining to arcade game ports at the time.
First off, the rapid growth of home console and computer gaming in the early 1980s was directly associated with the rising popularity of arcades. The arcade experience was the main gaming experience that people (kids, mostly) had prior to this time. And popular arcade games like Space Invaders, Asteroids, Centipede, and (of course) Pac-Man had already captured the attention of kids in every town large & small that had an arcade. The entire promise of the early console/home computer gaming market was in bringing this "arcade experience" into the home, minus the quarters, and preferably with the games that kids already knew and loved. It was distinctly NOT about introducing the public to unfamiliar computer games that had been around academia for over a decade. Those types of games didn't have the market clout of the popular arcade games, and wouldn't convince a mass market to invest in consoles of home computers at the time.
It has been argued by early game console historians, I think quite effectively, that the reason for the early growth of the Atari VCS platform was that Atari delivered a credible, playable, fun port of Space Invaders for the platform. Before that, it was not clear why the mass market would buy into owning a VCS. Space Invaders was immediately recognizable, and the ability to have the same basic arcade experience on a device at home was what sold the mass market.
So, if you wanted to sell a successful home gaming platform at the time, your best marketing advantage was the availability of good quality ports of popular arcade game titles. And, to make this truly effective, you would want the home version of the game to share not just the game play aesthetics, but also the NAME of the arcade title. This is where trademarks, and trademark infringement comes into the focus.
Trademark infringement was already a very-well established legal framework at this time, and was easily applied to arcade game titles. Copyright was not so relevant initially, because software was something new, and it wasn't yet clear how to correctly apply copyright beyond just not being allowed to pass off someone else's source code as your own. The rule for Trademarks is simple: You cannot use someone else's Trademark on your own product, especially when your product purports to be the same thing as the other's product. So, creating a "Space Invaders" video game using that trademarked name (without permission) was out of the question. You would likely be slapped with an injunction from the Trademark owner, and then they would come after you for all your profits from the infringement plus punitive damages. It was simply way too risky and potentially business-destroying to infringe the trademarks of popular arcade games.
This situation begs the question of how to make recognizable, high-quality ports of popular arcade titles available for your budding home gaming platform, given that all such arcade titles were protected by trademarks, and infringing on the trademark was legally untenable. The solution pursued by most publishers (Acorn, Commodore, small third-party) was to make a knock-off of the arcade title instead. Clearly, you could not use the trademarked name for your game, so you made up some new name that sort of sounded related while offering plausible deniability. Then, to farther strengthen your plausible deniability, you changed some details about the games visuals and back-story. The essential game play would remain the same, thus capitalizing on the players' main affinity to the game. So an actual gamer would still recognize the game and want to own it, but the trademark infringement attorneys would have a much harder case to make in court.
This "loophole" was exploited for several years. Some legal claims were made here & there against those trying to exploit the "loophole". Usually these cases were easily settled since neither side had a very strong case and the money involved was usually not much.
At the same time, the larger publishers with the deeper pockets simply struck trademark licensing deals and legally created ports with the same name as the arcade title and with audio/visual content matching as closely as possible. It turned out that these gaming platforms were the most successful. The follow-up was that more makers of arcade games got into the home video game market to offer their own ports. This allowed them to maximize the lifespan and profitability of their IP.