If the trust (really, the trustee) has at least one employee (including a person who is a deemed employee under IRC 401©) who might become a participant, I'm unaware of any special reason why such an employer could not establish a retirement plan.
There can be non-nefarious reasons why a trust creator might want a trustee to operate a business.
If the corporation has S corporation flow-through treatment and the trust would be a grantor trust that has the trust's income taxed to the grantor, it's possible that the Federal income tax treatment might not be too much different.
tbp, consider being extra careful about the scope of your advice or service.