What's confusing about it? A recall in the automotive world has a very specific definition, and it covers not only software related issues but hardware related ones as well.

The National Highway Traffic Safety Administration (NHTSA) is a part of the US Department of Transportation, and they publish a 20 page pamphlet that describes what a recall is. Here are the relevant parts from that brochure:

The United States Code for Motor Vehicle Safety (Title 49, Chapter 301) defines motor vehicle safety as “the performance of a motor vehicle or motor vehicle equipment in a way that protects the public against unreasonable risk of accidents occurring because of the design, construction, or performance of a motor vehicle, and against unreasonable risk of death or injury in an accident, and includes nonoperational safety of a motor vehicle.” A defect includes “any defect in performance, construction, a component, or material of a motor vehicle or motor vehicle equipment.” Generally, a safety defect is defined as a problem that exists in a motor vehicle or item of motor vehicle equipment that:

• poses a risk to motor vehicle safety, and

• may exist in a group of vehicles of the same design or manufacture, or items of equipment of the same type and manufacture.

Furthermore:

The National Traffic and Motor Vehicle Safety Act gives NHTSA the authority to issue vehicle safety standards and to require manufacturers to recall vehicles that have safety-related defects or do not meet Federal safety standards.

In other words, federal law gives NHTSA the authority to issue recalls for any defect that is considered a safety defect. There is no qualifier for it having to be mechanical in nature.

I've had software-related recalls issued for both a Toyota and a Honda that I used to own. The Toyota one resulted in them sending me a USB stick in the mail and telling me how to install it in the car (basically plug it into the entertainment system and wait). The Honda one required a trip to a dealer to update the software in the ECU to prevent the cars battery from dying due to the alternator being disabled improperly. Just because these were software related in no way means they weren't recalls. They were both mandated by NHSTA, both resulted in official recall notices, etc.

Edit: Just for fun you might want to go to https://www.nhtsa.gov/recalls and do a search there. If you enter "Tesla" in the field for "VIN or Year Make Model" you can browse all their recalls. The very first one on this page is titled "Incorrect Font Size on Warning Lights". That's most definitely a software recall. It's assigned NHSTA recall #24V051000, and they list the affected components as "ELECTRICAL SYSTEM". If you read further it also shows the remedy was an over-the-air software update.