Is it against the law to promise long term employment to and then let them go before their probation period is up?

Although an employer has the right to terminate an employee for any reason at all (except for illegal reasons such as discrimination or retaliation), the employer cannot falsely promise you long term employment when he/she has no intention of keeping you for more than 90 days.  This is especially the case where you may have passed up another job offer to take this one based on what may have been a false promise.  These cases are not easy to prove, but it is worth meeting with an employment law attorney to further explore this, especially given your statement that the employer has done this to teh last 6 people he hired.