You are correct. Duration is not specific to the type of dosing instructions (simple, free text, taper, etc.). If the duration is set, then I would expect autoExpireDate to be set to start date + duration, regardless of what type of dosing instructions are provided. When provided, a drug order duration says “the patient should not be taking this medication after ,” which means the drug can automatically expire (not be considered active) after that duration has been reached.
So, it looks like you may have found a bug.
-Burke 
Ideally, we would pre-populate duration when possible
Duration is intended for drug orders that have a finite duration (e.g., giving an antibiotic for one week). Ideally, duration would be pre-populated when possible – for example, from a pre-defined order (i.e., you pick the “give seatoncillin once daily for a week” from a choice list and duration is its pre-defined duration) or by parsing non-ambiguous free text (i.e., you type the free text “take 1 tablet daily for one week” and an instruction parser is able to unambiguously parse the duration from it). If we actually had those features to infer duration, it would be pre-populated and the user could change it if they wanted. For lack of those features, we just provide blank duration fields for the user to populate as you describe.
Many drugs orders don’t have a duration
For many drug orders (HIV meds, blood pressure meds, diabetes meds, etc.), they are taken indefinitely and duration fields would be left blank (ignored).
Dispensing information might be informed/inferred from duration, but duration is not dispensing information
The duration is meant to be the duration that the medication should be taken/administered, and not dispensing information. In other words, if your pharmacy dispenses meds in one month supplies and you want the patient to take a medicine for 3 months and stop taking it, the duration would be 3 months, the dispensing information would likely be quantity for one month + 2 refills.