Der Infinitiv kann als Imperativ verwendet werden. Man sieht das z.B. auf Schildern ("Bitte nicht rauchen") und hört das auch in manchen ganz informellen Kontexten.

Like Clavin said, when a computer or a sign does this, it's probably meant to be impersonal and to feel less like a conversation. Note that it also avoids a "du"/"Sie" distinction while being shorter than a "Sie" form imperative (which on signs or in computer programs usually would also include a "bitte", too). So in such contexts, it's absolutely normal. If people do this... I think it's actually worse than the imperative. I feel like it implies a position of authority, even in a "du" context. Ten times worse with "Sie". It can be used in a non-offensive way, so there is not need to get offended right away - but that is with an appropriate tone and/or in the right situations. It can be used as some kind of joke, too, I guess? Disclaimer: This comment is based on feeling alone and I am just one person.