if thing A is a "front for" thing B, then thing A is acting as a disguise for thing B. The author of the sentence is suggesting that people or organisations promoting medical marijuana are often promoting the legalization and recreational use of marijuana.
the character or qualities that a person or
organization appears to have in public that are different from their
real character or qualities, and whose purpose is often to deceive
people or hide an illegal activity:
Don't be fooled by his kindness and sensitivity - it's just a front.
She presents such a cheerful front that you'd never guess she's ill.
The machinery company was a front operation for arms smuggling.
Several trading companies were set up in the early 1960s to act as
fronts for money-laundering operations.
Front (Cambridge Dictionary)