David Levine, Groom Law Group, answers:  

Excellent question!  A quick reading of Internal Revenue Code section 403(b) itself could lead to the conclusion that an Indian tribal government may not offer a 403(b) plan for any employees.  However, there are a few special rules to take into account:  

First, pursuant to the Small Business Job Protection Act of 1996, certain pre-1995 annuity contracts purchased by Indian tribal governments are treated as if purchased by a 501(c)(3) employer, and thus eligible 403(b) contracts.  

Second, since amendment by the Pension Protection Act, Code section 7871 and related Internal Revenue Service (IRS) guidance treat an Indian tribal government as a “State” for certain tax purposes including Code section 403(b) and thus eligible to provide a 403(b) plan for its educational employees.  

Getting back to your question, despite these special rules, it is generally not permissible to maintain a 403(b) plan for employees of an Indian tribal government working in a commercial business because they do not satisfy the definition of 403(b)-eligible employees.  State employees are eligible to participate in a 403(b) plan only if they are performing services for a public school.  Although there is transition relief that delays the time by which casino employees would need to be separated out into their own plan, it is not permissible to accrue benefits under a Indian tribal governmental 403(b) plan for commercial or other non-school employees for plan years beginning on or after August 17, 2006 (the date on which the Pension Protection Act was enacted).  See IRS Notice 2006-89. Further, it is questionable whether they could even accrue benefits prior to that date given their non-educational nature (subject to the special 1995 transition rule we have already mentioned).  

The Experts note that if you are looking for a plan for Indian tribal government employees, a 401(k) plan is likely the more viable option to consider.