If someone on here is reporting users in that manner, then that's pretty crappy. However, if they're using the company's artwork or even copying their trademarks, then technically the company is in the right.
Is it ok to clone a game? Depends on what you mean by clone. Just cloning the concept of the game mechanics but using your own artwork and storyline, then yes I'd say it's ok. But if by clone you literally mean clone the game to look just like the original, then that can be a slippery slope. For fun and learning purpose, I'd see no harm in it myself as I've cloned several games (Barbarians, Zelda). But definitely not selling it. The exception I think would be if the pre-existing title was abandonware or released open-source or under the CC-BY or something similar.
How much change is necessary to avoid being a "clone" is debatable. I think it depends on what makes it unique. Sometimes the graphics isn't enough, it's the level design and layout itself. If I made a snail that could pop into its shell and roll around like a ball at super super speeds on winding tracks, your first thought is going to be Sonic the Hedgehog, I guarantee it.
Trying to commercialize a clone can get you into trouble, like being sued by the originating company. For instance, a few years back someone cloned super mario brothers 1 and released it for free. It was web-based and playable on a browser. Nintendo went after him. I don't recall details, but in that case I doubt it was anything more severe than sending a cease and desist letter. But if he ignored it, then he could end up battling Nintendo lawyers, and we both know us common folk can't afford that battle.
I think inspiring from a game's idea or mechanics is one thing, even their art style. But if you have the same graphics or the same level designs, then you might want to not commercialize it.