Here it is, directly from the white paper:
If people could freely convert in both directions then traders could take advantage of the
blockchains conversion rates by trading large volumes without changing the price. Traders
who see a massive run up in price would convert to SMD at the high price (when it is most
risky) and then convert back after the correction. The Steem protocol protects the
community from this kind of abuse by only allowing people to convert from SMD to STEEM
and not the other way around.
The blockchain decides how and when to create SMD and who should get it. This keeps the
rate of SMD creation stable and removes most avenues of abuse.