At the time the blockchain was created, it was cited in the whitepaper that allowing conversions both ways added risk of abuse. I've talked it over with other witnesses though, and we don't seem to think that there is significant risk due to this, especially now that the 10% debt limit is in place.
If it was implemented, the conversions would be allowed regardless of the SBD price. Users would be unlikely to use them though if SBD was trading <= 1.
To state explicitly (which of course you know but others may not), the 10% limit was not in place in the original design/whitepaper. So the risks to STEEM holders and the ecosystem as a whole from increased SBD were much larger. It made sense to have a tighter restriction.