Not just a while. The US Patriot Act gave powers to allow border controls to hold people indefinitely without cause.
The US is a massive country that ~75 million foreigners visit each year. In 2018 that figure hit an all-time record high near 78 million, which is in curious contrast to your setup premise.
With so many people visiting the US every year, you surely can produce a very large number of examples of toursts being held indefinitely - for many years even, one imagines - without cause.