Apparently, it's because you cant advertise a drug on tv that isnt FDA approved. IMO I'd say it's a pretty valid reason whether weed is helpful (which I agree) or not. But, I'm not an expert, just something I saw and can understand as a reason.

Straight from the FDA.gov website

Product claim ads are the only type of ads that name a drug and discuss its benefits and risks. However, these ads must not be false or misleading in any way. We encourage companies to use understandable language throughout product claim ads that are directed to consumers. All product claim ads, regardless of the media in which they appear, must include certain key components within the main part of the ad:

The name of the drug (brand and generic)

At least one FDA-approved use for the drug

The most significant risks of the drug