"Non arrivare tardi!"

Translation: Do not arrive late!

[Jackie.Bowers]

Why are they using the infinitive form of the verb in this sentence?

[marziotta]

We make the imperative negative of the second person (not formal) with the infinitive.

It doesn't make much sense, but that's like that!

http://www.italica.rai.it/principali/lingua/grammar/topics/imperativo.htm