I looked into buying a (non-full fare) biz KE ticket earlier this month from ICN to SIN on koreanair.com, and they were asking for additional money to select (any) seats. I didn't press the matter or even bother entering my SM# (because the fare wasn't especially enticing), and that's my only data point, so I can't claim it's "normal". On the other hand, given this, it doesn't surprise me that the Delta agent essentially told you the same thing.