Is that exclamation point legal?

Yes, as a matter of fact, it is legal. Why? Because it communicates the writers' intent clearly in an eliptical construction that is being used as a paranthetical.

What's an eliptical construction?

It's a sentence with some of the elements left out -- but with the meaning implied in such a way that the reader can reconstruct the entire sentence.

Clicking on this symbol wherever it appears will take you to additional -- and often amusing! -- information about the subject being discussed.

"-- and often amusing! --" actually means "-- and it will take you to often amusing -- ."

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