Retirement tax questions

It seems unfair that I should have to take that risk, particularly since the administer, Fidelity, is the same.  I tried to buy back in using margin and was told that margin accounts are not allowed for retirement accounts even though this is a brokerage account, albeit a retirement one.  Why is it different for, say, equities when I want to move a stock from one brokerage to another with no penalty whatsoever?