Why do the french like cooking with cream? Why do aussies swear? Why do the poms have bad teeth? Why does all the world love a canadian? And hate the americans? Why do kites cost so much? And the lines always tangled when trying to launch quickly? Why is there always one in every crowd?
For the same reason a car has toe-in on the front wheels, to apply a little bit of pressure on each wheel to counteract any play in the steering and stop the wheels shuddering from side to side. On long fins the tips will shudder if they are parallel. On short TT fins there is no point :-). Another reason is the misguided (excuse the pun) belief that applying a little drag to the back of the board will make it track straight. The opposite actually happens as can be proved by taking a toy car, locking the back wheels and sliding it down a decline. See what happens, you will be suprised :-)
What board do you have? I would be surprised if they are pointed inwards. Most boards have fins that are straight. The outline of the board will actually give you the illusion that they are pointing inwards in most cases. Take a straight edge and put it from fin to fin, this should tell you for sure.
Fun fact - Technically "fins" on a TT are just stabilisers.
p.s. I think TT's with small fins should actually have fins with a slight splay (like on surfboard) as then when you edge you would be using the fin at it's optimal angle, instead of loosing grip.
