Yes, in principle, "Bewahren Sie..." is possible. The other construction (without "Sie") is called an "Ersatzform des Imperativs" (i.e. a replacement form of the imperative) using an infinitive construction.
First of all, this is perfectly good German. Moreover, it often shortens the sentences (see the examples of thekeyofgb or your own favourite examples), which seems to be one reason for its preference over the usual imperative construction.
Furthermore, but this might well be very subjective, the Ersatzform constructions seem more polite to me. Sentences like "Bewahren Sie Arzneimittel unzugänglich auf" are a valid order, however I somehow feel it would be polite to add a "Bitte" somewhere. If I say "Arzneimittel unzugänglich aufbewahren!", then I don't feel the need for a "Bitte" as much as with the other sentence.