In short, the error is shown when attempting to trace a process that has requested not to be traced.
The error isn't technically cause by iTunes, but by a process that has disabled tracing, using code like the following.
ptrace(PT_DENY_ATTACH, 0, 0, 0);
This code sets a flag on the process on the kernel level, which prevents debugging and tracing of the process.
The best-known application that does this just happens to be iTunes, the application whose DRM this API was probably invented for, but the API is available to other processes. I have seen a number of 3rd party application use this API.
Of course, like all DRM it can be broken. Options for bypassing this anti-tracing and anti-debugging feature range from using a debugger to skip the API call to kernel extensions that patch the feature out in kernel space.