- Credit card processing
- The largest marketplace with the most paying customers
- Download hosting
- Promotion
- Highly available document and key/value store infrastructure
- Keeping junk and malware out of the store, which maintains user trust.
I'd argue that, like most emotionally influence business decisions, Apple sticking with the same percentage is a good thing - witness how the markets vary when things are up in the air politically.
Keeping the same policies for a long time allows businesses to plan appropriately as they know what they're going to get, even if it's not ideal.