I'm devoid of sympathy for this TSO. That said, if he didn't really know what the pen was worth and thought it was just a pen on top of a filing cabinet, a felony conviction does seem a little steep. I personally think a misdemeanor conviction would strike the right balance. Had he actually taken the pen at the checkpoint knowing it belonged to a passenger, then a felony would seem like the way to go. Frankly, I'm okay as long as it results in some conviction and record.
If this the guy has no priors (most likely) and it is only the $450 pen then I would bet the offense will be pleaded down to a misdemeanor with no jail time.