Andrea writes, "My parents have a tourist visa to the United States and when they first came to visit they were given six months. They left before the expiration of their stay but they returned to the US after one month. This time, however, the immigration officer gave them only two months and also added on the passport "no EOS/AOS/COS." Why did this happen? Their return ticket is six months from now. We also had plans for them to stay here for six months and we don't what to do. Please help."
You see the idea behind a tourist visa is that someone visits the United States for sightseeing or spending time with family/friends or even conduct business like attending meetings with colleagues or trade shows/conferences. It is generally understood that someone with strong ties to their native country will simply not be able to leave their job and life behind to spend many months overseas, but the CBP agents are often kinder to senior citizens assuming that they are retired and have time to spend. In any case, it is understood that in a 12-month period, no visitor will spend more than six months in the United States.
In your case, it seems that your parents really want to live in America and visit their native country. This is clearly a violation of the intent of the visa and the immigration agents rightfully suspect that they are more likely to overstay. How much time a visitor is given depends entirely at the discretion of the agent and cannot be challenged by them. Assuming that they leave on time this time and try to return to the US within a matter of few weeks/months, it is highly likely that they will be given even shorter period of time or even denied entry and sent right back. The officer may also cancel their visas and that will make it extremely hard for them to get another visa or ever visit the US again. You must realize that tourist visas are not designed for an alien to spend most of their time in the United States (the thinking is also that many of these people then work illegally). If they wish to live most of the time in the US they should apply for permanent resident status.