piggybacking marketing; bandwagon sales method
A marketing strategy that exploits a current trend, boom, or event (including disasters) to promote and sell products or services. Often carries a negative connotation of opportunism.
After the earthquake, vendors appeared who used piggybacking marketing to sell masks at high prices.
That health food advertisement is piggybacking marketing that jumped on the diet boom.
便乗 means 'taking advantage of an opportunity or riding along with a trend'; 便乗商法 is the specific marketing practice derived from it.
悪質商法 is a broader term for malicious or fraudulent business practices, while 便乗商法 specifically refers to exploiting trends or events.
Compound of 便乗 (taking advantage of an opportunity, riding along) and 商法 (business method, marketing).