this is HORRIBLY misleading. to suggest that everyone that goes to america has to "convince" the border guard to let them in and to then say flat out, "they are not going to do that with someone who has "vague" plans" is simply a false statement.
You may wish to read the documents before you talk about the practice.
(3) Security and related grounds.--
(A) In general. Any alien who a consular officer or the Attorney General
knows, or has reasonable ground to believe, seeks to enter the United States to
engage solely, principally, or incidentally in--
(i) any activity (I) to violate any law of the United States relating
to espionage or sabotage or (II) to violate or evade any law prohibiting the
export from the United States of goods, technology, or sensitive
(ii) any other unlawful activity, or
(iii) any activity a purpose of which is the opposition to, or the
control or overthrow of, the Government of the United States by force,
violence, or other unlawful means,
Is working without permission lawful or unlawful? So if you give the immigration officer reasonable ground to believe that you're there to work, he/she is very simply not allowed to let you in. So if your reason for visiting is "Er, like, you know, see stuff." and you're of a profile which suggests that you may be likely to work illegally, you may very well find you have a problem.
but it still needs to be written down in the laws and regs that they can still refuse you because it simply is another security measure. if someone gets a visa and then while on the plane picks a fight with half the staff on the plane, lights themselves on fire, etc., well, they need to have it written somewhere that they can lawfully say, "you're not entering the USA".
Perhaps you would like to actually read INA § 212(a)? You'll find it goes into a fair bit more detail than "you're not entering the USA". Alternatively keep coming back and posting without knowing what you're talking about.