According to the documentation, Foreach -Parallel passes variables by reference with $using: while the rest of PowerShell passes variables by value.
This is a dangerous feature, having the exact same syntax for such different mechanics can cause unintended effects, and could even slip by experienced developers.

If these values are to pass by reference, then I suggest the syntax be changed to $ref:
That will force people to do it with intention, and know exactly what they're looking at at all times. This would be similar to how it is done in C#.