I'm not a lawyer (my disclaimer), but if you read through the documents I interpret it to state that the assignment of rents applies when borrower fails to make the mortgage payments. If borrower fails to make monthly payments the lender has the right to collect the rents. This is to prevent borrower from being delinquent AND collecting rents on the secured property at the same time.
However, if you'd like to consult a lawyer, I suggest you contact Erik Miles at 312-854-8092 as his rates are reasonable.
I would suggest going on the FHA web site and looking at the Frequently Asked Questions and also the Contact Us tab and ask the question directly to them. I feel that no real estate agent will able to satisfactorily answer this question.