Is it legal for an employer to not pay commission when I am terminated?

The law does not actually say when an employer must pay commission on when not--it depends on what the agreement was between employer and employee (that is, the terms the employee worked under). It is legal for the employee to be paid any earned but as-yet unpaid commissions after termination; it would also be legal for the employee to have to be still employed when commissons are actually paid to receive them. The critical thing is determining those terms, so you can see if they need to pay you the commission despite termination.
In determining what that agreement was, the courts (if you sue for the money) would in order look to:
1) A written employment or commission agreement/contract, if you have one;
2) If no contract, to some policy statement or employee, etc. handbook;
3) If no formal policy statement or employee handbook, written communications between you and the employer, especially those predating the decision to terminate you, regarding this subject;
4) If nothing relevant at all in writing, to past practice at this employer--that is, what have they done in the past when employees were terminated, which can show their policy; 
5) If none of the above, to industry norms--what does your industry generally do about this, since in the absence of more-specific guidance, the assumption would be the employer follows what the rest of the industry does.
Whatever can be reasonably shown to be the policy regarding commissions will control.