I think Trekker is on to something about twist ties being used for anti-theft than anything else as I've seen many a figure that after taking the 20-30 twist-ties out, the toy still won't budge!
My favorite example of blatant twist-tie use was with the Wizard exclusive Simpsons "Boxing Homer". He came with 2 boxing gloves. The wrist part (where there would be shoestrings on a real glove) had a notch so it could be put on and off easily. They each had a molded area in the bubble for them and the each had a twist-tie that went through the bubble, into the wrist hole and out the notch in the wrist.......that's right, you could take the glove out of the bubble and never touch the twist-tie!
So what is the purpose of twist-ties? I'm going with annoyance and anti-theft