I'm not entirely sure whether or not guest workers and implying scabs or immigrants, but either way both workers should be treated with respect. If they are working for lower pay and you feel that it is unethical than you should take it up with the government, but the workers should still have the right to work.
Guest workers are responsible for doing extremely laborious jobs, such as harvesting, that many Americans simply don't want to do. These jobs need to be done to make our country work in the way that we've become accustomed. Guest workers should be treated more fairly and be recognized as the valuable labor force that they are.
Given that general American workers are rarely treated well in the American work environment I see no reason to treat guest workers better. If the work environment needs improvement then it should be improved for all employees not just a small sub set. American jobs are spare and good ones are scarce.
Guest workers are already well treated here and there is no need to treat them better. Many are here illegally and should not even be allowed to work or even be here. Those that are here legally are p[aid better wages and have more legal protections than they would have in their home countries.