The limitations to at-will employment termination are usually found in various federal laws on discrimination on a prohibited basis such as race, gender, national origin, and disability to name a few.
Absent that you can be terminated for good reason, bad reason or no reason.
Nothing wrong with "at will"..employers want and need good employees although those needs can and do change. Just remember you own your CAREER and the company owns the JOB. Stay current on skills, save money and meet or exceed your employers performance expectations and you'll be fine.