Well, Data himself is wrong, then. He certainly uses them in performances and songs and in holodeck cosplay. As someone in the thread pointed out, from a programming perspective, it's very easy to get an AI to use contractions. I can't do much about the TV show or the fact that the writers couldn't stay consistent, but at least I answered the question for my own head-canon while writing a fanfic at someone else's request. I posited that the anti-contraction stuff was put in there by Soong, to differentiate Data from Lore, as an afterthought. It's why, when Data copies his neural net to Lal's positronic matrix, she can use contractions; Because Data didn't add the little bit of scripting to stop her from doing so.
Just my 2 slips of latinum...