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" In some instances, 為 and 是 is interchangeable:

"未經許可而出口黃金就為犯罪." ( exporting gold without permission would be committing a crime). 

"未經許可而出口黃金就是犯罪." ( exporting gold without permission would be "committing a crime).

Since '為' means 'be' here, my question is: What role is '之' playing in '為之'. As in "未經許可而出口黃金就為之犯罪"


Is 之 a modal particle just for emphasis?
Is '為之' being a compound word for ' be equivalent to'?
"未經許可而出口黃金就為之犯罪" ( exporting gold without permission would be equivalent to/ qualified as committing a crime).
Is '之' functions as an objective pronoun.?
"未經許可而出口黃金就為之犯罪"
"exporting gold without permission, it would be committing a crime."
Or it should be '謂之' (be called as) , not '為之'?


We are talking about '為之' in the context of '那就是為之犯罪', not in that of 好自為之/偶一為之, when '為' means 'do' and '之' being an objective pronoun;  or 為之氣結, when '為' means 'for' and '之' being an objective pronoun
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.