It's pretty straight forward:

"Nonplussed" is in the dictionary*

"Plussed" isn't. 

How is that possible?

It's plain to see the path forward.

Please, take appropriate action and join me in calling upon Nigel Portwood (CEO of Oxford University Press) to put things proper.

Our position in this matter is unwavering.

Put simply, it's a point of principle.

(*Non-plussed has also come to have opposing meanings on different sides of the Atlantic, but that's a whole other petition.)

Definitions and Examples:

Non-plussed (UK): To be perplexed by a situation to the point of a non-expressive standstill.

Non-plussed (US): To appear visually unimpressed through lack of reaction or expression.

Plussed (UK): Unperplexed and unfazed by a situation.

Plussed (US): Visibly animated/excited through excessive expression.

For example:

I was pretty nonplussed (UK) by the whole situation, I just didn't know what I could do.

But I was also really nonplussed (US) by it all, which lead me to start the petition.

You see, nothing leaves me quite so plussed (UK) as a well-formulated and clearly articulated plan for success.

Meanwhile, if this petition is successful, it would make me quite plussed (US).