It is not unethical. Ethics are the actions deemed acceptable by society. Laws are used to enforce ethics. The law currently allows for individuals to take advantage of these plans and does not preclude someone based on how they achieve a low income. Therefore, right now, it is not unethical.
Now if you are asking if it is moral, then it changes a bit. Morality is one's own rule-set for living life. Apparently, many on here believe it is immoral and have very strong views on personal responsibility. However, you can only use their opinions as a way to inform your own; the morality of the choice is up to you. Others can then value your actions as moral or immoral in their own view.
As for my feelings on the plan, if you don't mind having low income, regardless of how you go about achieving that, then I am all for the program. Educating a populace is a reward in and of itself. So if I have to pay for your college, I don't mind. Educating those around me, regardless of what they do with that education, benefits me. An educated populace is less violent, has a more sustainable birth rate, and has many other advantages. I like the program and I hope as many people as possible find a way for it to help them.