Actually it's currently an admin setting, your junk & trash folders by default are set to empty every 30 days.
This can be set per user or COS so ask your administrator (if your not) if you'd like your value lowered. It's at the bottom of the advanced tab > trash folder message lifetime, etc.
I know there's an open RFE to display a disclaimer on those two folders like: "Messages that have been in Trash more than 30 days will be automatically deleted."
Search
bugzilla to see if there's an RFE for setting junk/trash lifetime end-user side. (This also might be one of those things that could go along with active filters-in terms of where to implement it vs wasting space in a right click option)
edit: It's bug 13435