The same goes for many work situations where the boss suddenly lowers the pay or fires someone for not filling in last minute for another employee calling in sick on a weekend. How is that possible? Don't you Americans value written contracts?
I am often annoyed by the amount of paperwork needed to make my life go smoothly, from work to private matters to whatever - but the times I was glad I could just point to a written and signed, extremely detailed contract which is the only thing that matters should things ever get in front of a judge, easily make up for the tedious writing and reading of contracts beforehand.