I work in digital advertising technology and have for some time. There are multiple ways to place ads on specific websites some of which are automated through the use of software, some are through a direct agreement with the specific publisher. When using software to place ads the platforms allow users to configure a number of targeting parameters including, but not limited to, geographic area, demographic information, device, and of course, specific websites. The default configuration for the majority of ad tech platforms is to target everything and everyone and its incumbent upon the user to set the targeting parameters to meet the criteria of the campaign. If the advertiser has specific brand safety restrictions (in this case Juul most certainly should eg. not targeting children or sites popular with children) then it's up to them to adhere to those restrictions by using the advertising software correctly.
That being said, this is a complex industry with many variables. Advertisers often work with dozens of different vendors who execute campaigns on their behalf. These vendors are either using their own tech or a 3rd party platform to build campaigns, set targeting, and ultimately avoid any people/websites which should not be targeted with that brand's advertisements. For a larger advertiser this could mean many people at varying levels of experience operating hundreds if not thousands of individually configured campaigns all of which need to be set up correctly to avoid these sites/apps. It doesn't take much of a slip up at all at any level in this process for an advertiser to land in some hot water (see the great corporate ads before ISIS beheading video debacle of 2015). Exacerbating the issue, targeted advertising campaigns are often optimized toward a particular goal based on user engagement. If Juul was serving ads to children's websites and children were frequently clicking on them then the software itself would likely identify those websites as a winning strategy and automatically shift more budget to them.
I don't smoke, don't particularly like Juul, and wouldn't find it hard to believe that they are intentionally advertising to children. However, I want readers to understand how easy it is for something like this to happen by mistake given the current standards of the industry. If we think those standards should be changed perhaps that's a different conversation.
