As far as I can make out,
most employers limit their employees' selection of annuity providers. The only real limitation on this, in the case of an employer other than a church or governmental employer, is that the ERISA exemption under ERISA Reg. § 2510.3-2(f) can be lost unless the employer offers "a number and selection [of funding media] which is designed to afford employees a reasonable choice in light of all relevant circumstances."
On the other hand, the employer's prohibition may not be fully effective. Given the availability of nontaxable annuity exchanges under Code section 1035, an employee who wants to sign up with the prohibited company may be able to put the money with one of the permitted companies, and move it to the prohibited company immediately thereafter.
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