This is the Emergency Exit feature and is standard on F51 keyed entry locks. This feature cannot be disabled. The emergency exit or panic-free exit feature is required by some building codes for fire safety. Those same building codes will not allow double-cylinder deadbolts in some applications because they negate the emergency exit benefit of the knob/lever.
If you are not limited in choice by the building codes, you can choose a different function of lock. Schlage offers a keyed entry lock called the F54. The F54 is also a Grade 2 (as is the F51) with Grade 1 features. The inside thumbturn has to be rotated to the unlocked position before the knob will turn to open the door. If the store does not have the specific design/finish that you want in stock, it can be special ordered. It may also be available online.
If you would like to upgrade, you can also move to the A-series and order an A53, which allows both the Emergency Exit feature as well as the Unlock when the Door Opens feature. This lock is a Grade 2 and is also available as a special order or online.
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