Let’s look at the sentance fragment “It is not …”
We could say “It’s not …”
or “It isn’t …”
right?
why not “It’sn’t …”
is there a law against this? shrugs
Against the “law” as spake by Webster and Strunk and White.
And I like the “Conjunction Junction, what’s your function?” reference.
Actually “tisn’t” seems like it would be the (sub)standard word. Or, more appropriately, “tain’t”. Feel free to use these constructions, provided you realize people may start asking you what color your neck is.
And more importantly, tell me what sort of reactions you get to using the word TAINT in every day convos. 
jarbaby