They don't make anywhere near most of their money from branded products. The last year I can find easily is 2014. They made less than $500 million in revenue from merchandise and sponsorship, and $2.2 billion in car and part sales.
Margin is very high on their cars, so there is no way they're making anywhere close to most of their money through branded merchandise.
Contrast that with the Music industry. They did about $15 billion in music sales, and only about $8 billion in concert revenue in 2015.
Musicians do make a significant portion of their money from concert revenue, but musicians aren't the primary drivers of ticket prices.
If Porsche made most of their money from branded apparel, they'd probably have a different pricing strategy.