Normally I never attribute to bad what I can attribute to ignorance. However, I think your right. People tend to buy stuff on impulse. Imagine these scenarios:
1. Customer enters
LFS wanting to buy a 10-20 gallon aquarium and a appropriate number of fish. LFS owner explains that the customer will have a 4-8 week wait before he can start adding the fish he wants. Whim severely diminished man leaves with nothing.
2. Customer enters LFS wanting to buy a 10-20 gallon aquarium and a appropriate number of fish. LFS sells the aquarium, accessories and fish. Customer returns complaining of dead fish. LFS explains that fish in a new aquarium often have a high mortality rate. Offers replacement fish at reduced price.
Customer either:
a. Buys the replacement fish, 4-8 weeks later fish quit dying.
or
b. gives up
Unfortunate, but I'm sure scenario #2 generally wins.
BassNFool