Citing wikipedia's MAC article:
With mandatory access control, this security policy is centrally controlled by a security policy administrator; users do not have the ability to override the policy and, for example, grant access to files that would otherwise be restricted. By contrast, discretionary access control (DAC), which also governs the ability of subjects to access objects, allows users the ability to make policy decisions and/or assign security attributes. (The traditional Unix system of users, groups, and read-write-execute permissions is an example of DAC.) MAC-enabled systems allow policy administrators to implement organization-wide security policies. Under MAC (and unlike DAC), users cannot override or modify this policy, either accidentally or intentionally. This allows security administrators to define a central policy that is guaranteed (in principle) to be enforced for all users.
From what I've read, with MAC, the system defines security levels and with DAC, each user is responsible for assigning permissions to access its files based on user identity and not on security level.
In Unix, you cannot assign access to a specific user to a file based on its identity (considering only the rwx permissions with groups, and not ACL). You can't even change the file ownership to another user without elevated permissions. Besides, even if you could, the access is evaluated based on your "ownership" or "membership" (owner, group or other) and not on each user's identity. This also resembles more of a MAC model than DAC.
So why is linux filesystem permissions modele considered a DAC model?