Is this okay according to Missouri Labor Laws? It really doesn't seem right, and I can't find the answer.?

As unfair as it may be, the employer can do what they want unless your mom has a contract stating otherwise (which is highly unlikely since only top level management sign contracts nowadays). Your mom is an employee at will meaning they can basically fire her at any moment, no questions asked. So yes, they can also send her home without pay...happens in a lot of industries (nursing, food, retail) when business is slow they send people home...if she is unhappy maybe it would be best to look for a salaried job?