Marketers are split down the middle on whether tracking customers online without their permission is ethical. According to our most recent survey, 41% of marketers found the practice acceptable while 40% found the practice to be unacceptable. 19% of respondents either did not comment or were unsure about the ethics of the practice. The survey question did not cover the type of permission that's acceptable for the practice of tracking so one can assume that not all of marketers who indicated the practice as unacceptable require explicit permission.
Source: Determining Perceptions of Marketing Permission Impact Marketing Success