By allowing basically just anyone to come in and buy a pass to America's voting benefits, it diminishes the value of the citizenship, except the impact is greater than selling sperm or contractual surrogacy. Ultimately this comes down to what is best for the country be not allowing absolutely anyone to buy a ticket to being American. It's utilitarian, yes, but at this point in history national interest has to outweigh whatever benefit instant citizenship could produce. .
Now, if this question was in fact posed in the past, then potentially I believe the outcome could be different. Currently, the U.S. has a national identity and accompanying national interests; selling citizenship could put that identity and those interests at risk, especially during a period where there are those in the world who would wish to do America harm. Again, it's a strong utilitarian stance, but with a nation of over 300 million people there is a lot more at stake than the instant certification of those who wish to join the 300. .
However, if we were to look at denying the sale of citizenship in terms of border control, it can be viewed as immoral to not help those next door who need it. I tend to agree it is important to assist those who seek our help, yet I cannot get on board with giving people the ability to "participate in shaping the forces that govern the collective destiny " (108) of those currently residing in the United States. There is a reason the citizenship process takes as long as it does. Take it logically, where it can take anywhere from a few months to 20 years to obtain citizenship. A person born in the U.S. is a citizen for his or her entire life, but they do not gain access to the most important aspect, the voting process, until they are 18 years old. In Sandel's description of citizenship, "it is necessary that citizens possess or come to acquire certain qualities of character, or civic virtues " (108).