Zwarte Piet is 100% a slave with all the traditional symbolism around the character. The fact that people call their role 'helper' or 'assistant' really does not change anything about the historical background of "zwarte Piet"

We are basically portraying a white man with black slaves as a great thing, and celebrate it. Combine that with the fact that it's mainly directed at kids who are very easily influenced in such regards... It's a bad tradition.

I know that for 99% of the people this probably isn't the message they want to send, but even when not malicious it will influence the way children look at the world.