Warning: conspiracy theories ahead - I have a suspicion that the biggest reason is so that their field agents (i.e. oracle-friendly DBAs : ) out in the field can always find some reason to choose Oracle over anything else.
Also: and this is based on once working with a Oracle shop and also listening to Microsoft guys: licenses fees can be a blessing in disguise, they aren't going to pay them anyways, their customers will and they will add 15%
Provide enterprise level support. That is, they provide 24x7 support with technicians who can troubleshoot problems with the database or provide basic indexing support, etc. They also will write specific patches for specific customers as needed.
It also has more features than PostgreSQL (more indexing options, custom SQL extensions, a better optimizer), is more stable, and has a lot more developers working full time on it.
PostgreSQL is good, no doubt, and GAFA have shown that you can take an open source DB and throw engineers at it to make it handle ridiculous scale. But what if you don't have those engineers to throw at the problem and constantly patch/update the DB? Finding people qualified is going to be both hard and expensive. An average MySQL DBA can easily fetch $200 an hour for routine work, and over $400 an hour for emergencies (my numbers are about 4 years old; it's probably gone up), and never be out of work or job offers. I don't imagine a good PostgreSQL admin is worth any less.