I believe the argument is that the passthrough fees can vary by location in a way that makes it hard to produce a precise fee calculation on promotional material. As a random example, it's plausible that a local government could charge a fee for installing new hardware in historic buildings; failing to include that fee when advertising to residents would fall afoul of the previous rule.

Of course, the counterargument is that ISPs should just be eating charges like that rather than passing them on to consumers. If they don't want to have to advertise confusing, long fee schedules, they shouldn't try to charge them in the first place!

Could they not comply by means of a footnote pointing to some fine print somewhere which lists all the possible charges there could be along with possible conditions leading to those charges?

That's in the spirit of passing on. Pass on the charges to subscribed users, and to prospective subscribers, pass on the work of figuring out what extra charges apply in their location and situation.