there is a simple reason for that. Economics is practically useless for IB, just like any other degree. Hence you need no specific degree to go into it. ( so that answers the first question, you dont need an Eco degree to go into IB).
However, (and this is a big however), there is a vey very good reason why IB's like ppl with Eco degree - the kind of preparation it gives you. I mean you have to be both mathematically able and able to write well. I mean most subjects are either one or the other (science or art). Eco combines both, so you become a really flexible person, and Eco studies give you the skills useful in the IB industry.
Now, if an individual posesses all those skills having done Biochemistry(or anything else) at University (as a family friend of ours did and went to Citibank) then he will have no problems. But Economics develops them if you dont have them.