Exempt employees are expected to get their work done, but the work does need to be reasonable. If they give you 40 hours of meetings you can have a good case they are asking too much to expect anything more. While hours are not given by law, there is still an expectation of reasonableness.
Which is to say they cannot fire you for not getting your work done. However at-will means they can let you go - but that is not firing you for cause and there is a big difference in how the law treats that.