What is the role of '之' in '為之' ? As in "未經許可而出口黃金就為之犯罪"
Odd example, knowing where it came from would help. It is definitely not classical Chinese. (就 is never an adverb in classical Chinese and who would write classical Chinese in this context?) It is also definitely not Mandarin. As the OP observes, it is frequent in Cantonese. Alas, my Cantonese is almost non-existent. But, looking up the phrase in the Cantonese Wikipedia, there are numerous examples which seem very similar:
相撲係日本傳統運動，由兩位肥獅大隻嘅人紮馬對打，推跌對方就為之贏
最後邊隊入得多啲就為之贏
手牌入面明暗牌同其他人捨牌(含加槓)或者本身嘅摸牌組成「五搭一對」就為之胡牌
進攻一方如果有三名球員出局嘅話，咁就為之完成一局（inning）
etc... So for sure it's not a careless mistake.

It seems to mean the same thing as 叫做 or the more literary form 稱之為. (Or perhaps it is more like 算是...?) 

As for the grammatical structure, 為之 must be v-o. 為 is very unlikely to mean 'be' here, since 之 is never used as a 表語 (predicate of a copular sentence). Functionally, 之 is being used as in 稱之為; it is the object of a verb.

Summary: looks like a Cantonese way of defining technical terminology.