First of all, the proverb is " All Is Fair In Love and War"
I believe...that there can be a reason for every proverb that has gone through our eyes..nothing is useless when it comes to reading...and when it becomes a saying ...then obviously it has a history too...lets find its origin...the base root of this proverb.
The Sources in History
1)from Chaucer in Troilus and Criseyde (c.1380). But, the saying has a long history in various forms that justifies cheating:
'But now ne enforce I me nat in shewinge
How the ordre of causes stant; but wel wot I,
That it bihoveth that the bifallinge
Of thinges wist biforen certeynly
Be necessarie, al seme it not ther-by
That prescience put falling necessaire
To thing to come, al falle it foule or faire.
~excerpt from Troilus and Criseyde, by Geoffrey Chaucer
there are other sources in which he didnot exactly used these words but had almost similar meaning...and to tell u the truth...there is no origin of the thought...only variations presents as a guide to know...
'But wene ye that every wrecche woot
The parfit blisse of love? Why, nay, y-wis;
They wenen al be love, if oon be hoot;
Do wey, do wey, they woot no-thing of this!
Men mosten axe at seyntes if it is
Aught fair in hevene; Why? For they conne telle;
And axen fendes, is it foul in helle.'
~excerpt from Troilus and Criseyde, by Geoffrey Chaucer
2)1579-John Lyly in Euphues: Both might and mallice, deceyte and treacherye, all periurye, any impietie may lawfully be committed in loue, which is lawlesse.
http://www.geocities.com/Athens/Aegean/2771/quotes.html
3)1623-Fletcher and Massinger in The Lover's Progress, Act V, sc. 2: All stratagems/In love, and that the sharpest war, are lawful.
http://www.geocities.com/PicketFence/7608/sayA.htm
4)1677-Aphra Behn in The Emperor of the Moon, Act I, sc. 3: Advantages are lawful in love and war.
http://list.gatech.edu/archives/lcc3303o/0082.html
http://www.inform.umd.edu/EdRes/Read.../Behn/oroonoko
5)1835-First appeared in U.S. in Horseshoe Robinson.
http://www.ags.uci.edu/~ishmael/makingan.htm