It is more a matter of Mass Mutual wanting to hold your money. I went into management 3 years ago and am no longer allowed to participate in the Teamster 401K. I tried to roll that 401K into my new 401K but was told I couldn't. I explained that I was no longer a Teamster but was told as long as I work for Roadway (YRC) I can't even though I am not eligibale to contribute. Mass Mutual is a POS company, IMO!
I still do believe that's a government rule in my opinion. You could check for sure with a good financial advisor or CPA.