One-time offers are a marketing tool not only used on the Internet.  It's just that they can be done a lot more efficient on the Internet as an offer can be much easier removed after the first view then that is possible offline.

Offline e.g. a Presidents Day sale is comparable to an online one-time offer.  Once Presidents Day is over, so is the special sale.

Also, so called up or crosssell are done offtine as well.  When you order at Mc Donalds, they used to ask you if you like to "Super-size" your order, which is an upsell (more of the same).

They also do crosssell when they e.g. ask you if you "like fries with that" or "would you like an apple pie with that?".

Whenever you see a one-time offer, an upsell or crosssell online... just remember, it's optional and your choice to take advantage if the special offer... or not.

As you can see... there is nothing immorral about it.