Why do they cut pigs' heads off and sell them??

Does anyone know why butchers cut off a pig's head and sell it? All I can say is that it is disgusting. My advanced spanish class went to a market this morning for class. Don't ask me why we did because none of us can figure it out. But, I was very closter-phobic in a short period of time because there were tons of people in this market which was in a building and not in an open-air market. So, we walked by tons of fruit and vegetables as well as food stands were lots of people were eating and nothing was clean. We then walked by stands that were selling spices that had a very strong smell. Walking by the first stand was not bad, but after walking by them multiple times in a matter of 30 minutes I had had enough. On top of that, we walked through the meat market part of it which was disgusting and I have never liked them anyway. BUT then I actually look at what I'm walking passed since I was looking to the opposite side of the market beforehand and saw that I was walking by a pigs' head!!! I just about lost it. I love animals and I couldn't handle it. However, our professor thought it would be ok to stand by this area and answer some of our questions instead of going outside. We then proceeded to walk through the second level of the market which wasn't open yet so we had to walk back by the meat market, then through the chicken butchering area, then by more spices that were overwhelming. While all this was happening, I looked to Danielle who is in my class and she was about ready to lose it too. I told my professor that we needed to get out of there but she laughed and took us by more fruits and vegetables to explain what they were. We were absolutely mad at this point. Did I mention she had never been there before and didn't know where she was going which meant she didn't know how to get out!?! While walking further to ask someone how to get out, we walked by a stand that was frying whole fish and WHOLE birds. The wings and everything. I was grossed out and almost got sick when next to this stand were cages of birds and chickens and puppies and bunnies. I got so mad that they had puppies in cages and then I about broke down and cried walking past the bunnies that were not being taken care of like I took care of Oreo my bunny, that I lost a couple weeks ago. As we found the exit and were walking toward it, our prof. thought it would be ok if she walked at a snail's pace. I was about furious at this point and I hardly ever get that way. I was definitely going through culture shock through all this. This is the only time I have really had it since being down here. This market definitely fulfilled the stereotypes Americans have for mexicans even though our stereotypes are completely wrong. This morning was just way too much in so little time. So many smells and images that made my stomach churned. I'm surprised I didn't come out of there deciding I wanted to be a vegetarian! Smells and images of food normally make me not able to eat them, so I just pray I don't think about that experience anymore! Did I mention that there were tons of people in there that definitely had body odor and tons of sweat?? Then they wiped their sweat onto their palms and proceeded to pick up fruits and vegetables to sell......I have vowed to never go there again!! It even affected me so much that I needed to leave the Patronato a little early. But, I was also able to leave early because there was nothing going on. Luckily, the culture shock and subsided, but I never want to go through an experience like that again.