You can't have two imperative verbs in a row, just as you can't have two present-tense verbs in a row. Like in the sentence "mi volas esti alta" (I want to be tall). If you tried to say "Mi volas estas alta", it would be wrong. In the sentence above, "be" is subordinate to "please" so it must take the infinitive form. If you really want to say "estu" you could rewrite "please" as an adverb and the sentence would be "Bonvole estu atenta". This form is less common but it is grammatically correct. "Bonvolu esti atenta" could be translated as "Be so kind as to be attentive" where "Bonvole estu atenta" would be more like "Kindly be attentive".
Because, in the imperative form, if the subject is not mentioned, it's understood to be
"vi estas atenta" -> "vi estu antenta" -> "estu atenta"
This is different from the impersonal verbs, which don't need a subject at all:
"estas varme", "necesas, ke ...", k.t.p
I hope it helps. :)
I would understand it to mean both "Please be careful" and "Please pay attention", but I also don't think they have that different of a meaning, per se. If you want to say "Caution" you could say "Singardo" and "Danger" is "Dangero", so if the shades of meaning are important to you, you can use those words instead of atenti: "Esti singarda!"