Is it illegal to enforce a mandatory meeting for an in-service without providing compensation to the employees?

Yes. If you are an hourly, non-exempt employee, then you should be paid for this time. If your employer is doing this, there might be other ways you are not being paid properly. You should contact an Ohio employment attorney.   Ohio wage and hour attorney http://mcoffmanlegal.com/unpaid-wages-and-unpaid-overtime-attorney/