How can trademarks exist for seemingly similar graphic designs?

Hi! I was at a craft festival in Maryland this weekend. I noticed a surprising number of artisans using the shape of a Maryland blue crab with a Maryland flag in them. There were paintings, stickers, clothing, etc. A number of them had the characteristic “TM,” but the different creations were clearly not related. How can this be? I guess I just don’t understand trademark law very well. If you google “maryland crab flag logo” you get this http://goo.gl/Q4Hcbz How do they all do it? Is it because the “crabs” are all a little different? It just seemed odd to me, they are all so very similar. Thoughts?