It's a mixing of the English language. Appendages at the end of legs, one is a foot two or more are feet. Measurements are different one foot, two foot, Three foot etc. So 6,ooo foot would be correct. People just mixed the two up. However like many things in the English language it has been accepted and now both are right.
Having said that, I have no idea why South African pilots say foot. Perhaps it is just because they can..
