willful deductions of wages without consent IS illegal.
Place I used to work at had that policy; till shortages came directly out of the wages of staff who were working that till. Consequently, I was always very careful to count the till on staff handovers, and not being an innumerate scrote I never had till shortages.
Got my wage one day to find that I'd been docked a till shortage. Kicked off, management disinterested. Long story short, I broke into the office and went through the close sheets for that day. The store manager had forged my signature on the paperwork. Being young and naive, I quit shortly afterwards rather than suing the barstools.
I should add, this wasn't some small business either, it was a large national chain.