Slave owners universally don't want to be slaves, thus it's immoral. Buyers and sellers of proprietary software don't have no problem buying and selling proprietary software from others, thus it's NOT immoral.
There're also plenty of examples of businesses trying to buy as cheap as possible while at the same time trying to sell as expensive as possible. That just proves that buyers should beware.
Also, proprietary software is not equivalent to high switching costs: in fact, it's a rather subjective calculation. If you don't care about keeping your old emails, switching your E-mail client has nearly no costs, for example. Additionally, "free" software doesn't imply low switching costs: develop a large application using the KDE or GNOME APIs, and you'll find that switching is rather expensive.
In general, your argument is a red herring: Since switching costs may establish a trap, you should warn about software with high switching costs, not about proprietary software.
Actually, most people don't know a thing about the issues around proprietary and free software. If they did, I am confident that most (more than 50%) would rather buy free software.
Now of course those who sell proprietary software happily buy proprietary software: they often don't have a choice. They can't use GPL software. They can however, use BSD licensed software. And I bet they prefer that over proprietary products.