Yes, because memory is a factor, and the LEM will prioritize processing new events and keeping rules firing over the database partitions being warm. Our assumption is you want your real-time alerts more than you want to search for things.
As far as "removing" alerts, the LEM is passive. It's like a mailbox: it doesn't go out and get mail, it just takes whatever you get delivered. If your mailbox is full of chain-letters and magazines, you should consider reducing the number of pen-pals and subscriptions you have. If you've got a lot of alerts that you don't want or need, kill them at the source!
Ironically, I just posted a thing related to that... http://thwack.solarwinds.com/docs/DOC-174865