This is something I had not thought of. Why do they state that drain
cleaners should not be used in toilets???

The reason for the warning is that drain cleaners can cause a lot of
heat and that could crack the porcelain.

http://www.drano.com/en-US/Pages/FAQ.aspx
The only Drano® product recommended for use in a slow toilet is Drano®
Max Build-Up Remover. When used according to label directions, the
microorganisms in this product will break down toilet paper and organic
matter in pipes, which can slow water flow. (This product will NOT open
a completely clogged toilet.)

Do not use any other Drano® product in toilets. Drano® Kitchen Crystals
Clog Remover generates heat that can cause the vitreous china in the
toilet bowl to crack. Drano® Liquid Clog Remover and Drano® Max Gel Clog
Remover do not contain microorganisms. The trap configuration in toilets
prevents the Drano® Liquid Clog Remover and Drano® Max Gel Clog Remover
from reaching areas where matter can accumulate and cause plumbing
problems—so these products are ineffective in toilets.