bitUSD is not destroyed. Collected field goes into yield pool which interest is drawn from.
After this post, rivaled by only by MY spelling ability, let's shed some light on the issue.
In general bitUSDs collected by the system as fees, go into a 'fund'. From this fund the yield/interest is paid.
Depending on which market engine we are talking about (v1,v2, or upcoming v.3) different fees go into the yield fund.
[PS] And as toast explained, if anything is burned it is not bitAssets (bitUSD), but only BTSX.