Btrfs merged for 2.6.29

o There's more Oracle products than their famous database. Some of them are relying on the underlying filesystems performances.
o Oracle do sells and support Linux already. So they already have to officially support filesystems, and to have experts able to improve/fix filesystems. They may find economically relevant, in the long run, to have one filesystem they know perfectly which covers all moderns use cases for their customers, and which is well designed for robustness (ie. to work around modern storages deficiencies).
o I don't about you, but, as a sysadmin, until now I would hardly choose to deploy "Oracle Linux" because I had serious doubts about their expertise there (compared to, say, Red Hat). The huge momentum around btrfs shows me that Oracle is serious about Linux things. If anything, this is an efficient promotional decision (and two fulltime developers is not a high price for this, I believe).
o Databases do not sells as single, isolated, products. They are to be used in larger datacenter setups, where you may find app servers, backup servers, web servers, etc. using the database. Databases are linked to the whole ecosystem. And when your ecosystem is built around Microsoft products, their own database product (SQL Server) has an edge. If, compelled by shinny ZFS features, your built an ecosystem around Sun/Solaris, you may also consider using their own database product (MySQL), and so on.

This is not an imperatively coupled choice indeed, but your internal expertise (sysadmins, developers, tools) will grow toward your infrastructure (including operating system) choices. Those choices will already bring you commercial contacts, and loyalty incentives (discounts...). You will already have paid support...

So yes, Oracle has interest in making Linux (which is - their own distribution anyway - also an official Oracle product), an attractive, compelling option, even if this does not benefits their main database product so much ; if anything to counter Microsoft and Sun in DC.