The CBP’s original position on the matter was "as marijuana continues to be a controlled substance under United States law, working in or facilitating the proliferation of the legal marijuana industry in U.S. states where it is deemed legal or Canada may affect admissibility to the U.S."
"A Canadian citizen working in or facilitating the proliferation of the legal marijuana industry in Canada, coming to the U.S. for reasons unrelated to the marijuana industry will generally be admissible to the U.S."
While the new statement is more permissive in its language, they still caution that "abusers" of drugs banned in the United States, including marijuana, would be cause for a ban. As any level of use of these drugs could be constituted "abuse," admitting you smoked marijuana could still potentially get you barred from entry.
No other announcement has been made, nor has a reason for the change been given by officials.