My question is about the situation around student visa F1. In the application for F1 visa document called I-20, one reads and signs under a statement that he/she wants to stay temporarily in the United States for the purposes of studying in a university.

However, everyone who finishes a university with an F1 visa can, and many (tens of thousands of people every year) do, apply for jobs in the United States, many find one and stay on with H1B visa which is a dual purpose visa, meaning you can legally pursue immigration into U.S. or go back home after expiration of your H1B. This is in addition to 12 or 18 months of Optional Practical Training period allowed to F1 visa holders.

Now my question is: if the purpose of F1 visa was to provide a temporary stay in the U.S. for the purpose of study only, how come people can change the visa type to H1B and stay on? I do not suppose tens of thousands of people every year (probably already millions of people over the decades) could fall into a legal loophole; or this could not be an inattentive immigration officer making a mistake - again, this is happening millions of times over the many years.

For comparison purposes, a seemingly similar situation exists with J-1 visas (a visiting/exchange scholar's visa), which has a two-year-home-country return requirement. However, among foreign students J1 visa is considered much 'worse' compared to F1, because everyone knows that with F1 you can stay on and there is no problem - there is a clear, million times passed path through H1B to Green Card; while with J1, the U.S immigration does follow up, and anyone who in the past has held a J1 and has not stayed back in home country for two years, will have huge problems when applying for a Green Card. Not so with F1 - there seems to be no problem at all for F1 visa holders in this regard (there may be other difficulties, but my question is not about them).

So, what does all this mean - does the above-mentioned statement in I-20 mean anything? If nobody ever follows up with that, why is it there at all? If it is there for a purpose, what is that purpose? I'd appreciate any comments - even if you are not an expert in immigration, but are a legal expert, perhaps you could give me an argument from a legal philosophy point of view or something. Thank you.