Ineptness in State & County government

[ElysianWort]

There's an Ad on the radio, playing currently that says if you pick party A and vote for someone in party B it won' count. Why do we have to pick A or B?

"I have no idea where you got the idea you have to vote a party slate."

Edited to try to clarify ad:
I think it's the same one that talks about voter registration and how, now for the first time, you can register the same day at the place you're supposed to vote.

(maybe I'm just trippin' here but I swear I didn't make it up)