This doesn’t exactly make sense, since all Kosher foods, and probably Halal foods should already be labeled as such.

Halal food isn’t. This was a big thing in the UK a few months ago when it was found out that Halal meat was being served (unbeknownst to consumers) in many schools and hospitals, and sold in all major supermarkets without being labelled as such. Given the option, most Britons would not eat meat slaughtered in this manner and the tabloid media seized the opportunity to malign all religiously slaughtered meat as part of some covert sharia-takeover.
Sharia conspiracies aren’t the preserve of americans