Sometimes such ‘infringements’ are considered parodies.
Copyright law has a parody exception; you cannot act against a copyright infringement when it is clearly meant as a parody.
Trademark law does not have such an exception. However, there is always freedom of speech, that can sometimes prevail above the exclusive rights of a trademark owner.
Suing someone can attract attention. Negative attention. There are cases where, as a trademark owner, you just have to let things go, and wait until it blows over.
As the WWF did with this parody on the diminishing panda population.
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