Very interesting question, Sarah. It's cool to see that you're interested in Judaism. Let me answer your question my way.
Jewish law is based on the Torah that is contained in the written torah as well as the Oral Torah which is a collection of traditions dating back to Moses as well as later rulings and enactments by Jewish judges and the Sanhedrin.
We follow both the traditions dating back to Moshe as well as the later enactments because by doing so we fulfill the words of Torah itself.
Allow me to quote: Deut. 17:8-11
If a matter of judgement is hidden from you, between blood and blood, between verdict and verdict, between plague and plague, matters of dispute in your cities -- you shall rise up and ascend to the place that the L-rd, your G-d, shall choose. You shall come to the priests, the Levites, and to the judge who will be in those days; you shall inquire and they will tell you the word of judgement. You shall do according to the word that they will tell you, from the place that G-d will choose, and you shall be careful to do according to everything that they will teach you. According to the teaching that they will teach you and according to the judgement that they will say to you, shall you do; you shall not deviate from the word that they will tell you, right or left.
What possible knowledge is there that can be hidden? If there is no oral law, then the only basis for judgement is in the Torah which is open for anyone to study.
Clearly, the entire need for the above process of going to the central court and following their ruling implies that there is an oral tradition which also serves as the basis for judgement.
Now, if a Jew decides that he would rather not carry, he may do so. The Eruv simply permits one to carry. It does not forbid someone to decide not to carry.
As regards to the "cheating" aspect, there are many aspects of Jewish judicial ruling that certain people beleive is like "cheating", such as women covering their hair with a wig. Many authorities say that this is "cheating" and therefore forbid the wives of their communities to do so. But they cannot say that a Jewish woman is doing something evil in doing so because she is simply following the ruling of different authorities.
When there is an actual Sanhedrin, all Jewish people must follow the rulings of that one central Judicial body. But today, each community follows it's authorities.
Some groups of Jews, mostly Chassidim, do not use an Eruv. So there are Jews that have your position on the matter. But they should not attempt to stone Jews who hold differently.
Until a true Sanhedrin is established, there will continue to be alot of diversity of custom and law among the Jewish people.
I hope that I answered your question to the best of my ability.