Does yahoo get kickbacks for allowing spam pharmacy email to "get through" their spam filters to email users?

Seems odd that remotely questionable email and often times legitimate mail gets filtered out, however, everyday there is mail coming through under the guise of, for example, "Betty Johnson", with solicitation to buy prescription drugs from Canada. The language used by these emails are so obviously for prescription drugs, even the simplest of filters should catch them. With words like, 'pharmacy', 'drugs', 'meds', etc. Since I'm a paying user (20.00 a year for additional features including spam filters) I would like to know if they are generating additional revenue on emails such as these. Thank You.