I'm not sure how it's arbitrary. If it's a lambic-style with fruit, it's a fruit lambic. If it's a classic style (like a Berliner Weisse) with fruit, it's a fruit beer. If it's a non-classic sour with fruit, it's wild specialty. All that being said, you enter it on how it comes off. If it tastes like 3F Oude Kriek, enter it as a fruit lambic.
Once you add fruit to a sour wheat beer the distinction between that beer as a lambic, berliner weisse, or wild ale (whatever that means) is greatly diminished because the fruit is going to mask/change some of the flavor in the base beer, change the acid profile and potentially change the ABV. What's really different about a berliner weisse with cherries and a 4% ABV barley/wheat wild ale with cherries? Do you think the guidelines clearly explain to an entrant how to distinguish between those beers?
It seems ideologically inconsistent with rules that, for example, let a session IPA, a black IPA and a white IPA all compete against each other. There you have fairly different beers that all compete against each other because they share core attributes but here you have fairly similar beers that are spliced across three separate styles. The explanation for the distinction is procedural rather than substantive but I don't see a good reason why sour beer suffers under the weight of these procedural rules while other styles do not. I believe there are far more judges who can distinguish the nuances between a session IPA and a white IPA than there are judges who can distinguish between a berliner weisse with cherries and a lambic with cherries and a wild ale with cherries.