It's colloquially used in a sense that suggests it's legal or a legal gray area but it's also important to point out that there's no such exemption. "Mundraub" is simply minor theft (like shoplifting low-value products) which means you won't be prosecuted unless the injured party presses charges.

"Stealing" fruit (or flowers) from public flora is legal in a practical sense and under certain circumstances explicitly permitted if it does not involve trespass, you only take small quantities (which is not legally defined) and you do so carefully (which is also not legally defined). There's no requirement for immediately consuming the fruits on the spot (as the colloquial use of the term "Mundraub" suggests).

As with most of the things laypeople think of as being legal, it's more of a case of how much someone cares to enforce the law that makes it illegal. Also note that "public land" may not actually be public despite being publicly accessible. A lot of former nobility retained their land despite losing their titles and it's not always clear that this land is actually privately owned, especially if you're not from the area. Some is even tended by municipal governments as part of contractual agreements for allowing access to the public. Germany did not get rid of its nobility like e.g. France did even if we officially no longer recognize titles.