I don't know how it is determined that it is illegal. I searched USPS web site and could not find clearly spelled out words that say it is illegal to ship any particular photographic chemical. If one ships one and if it leaks the shipper may face a steep legal battle. It is unclear how steep such a battle may be.
Originally Posted by tiberiustibz
The funny thing is when I last visited Chang's Photo Supply (a chemical/paper supplier to California processing labs) they had a van fully loaded with chemicals ready to make their daily delivery all over the San Francisco Bay Area. It is of course legal to ship such high volume of chemicals in a van that is not any different from all the vans running in the streets. It is only illegal when you pay the government agency (USPS) to ship a few small bottles of the same thing. I guess their large fleet of USPS trucks and vans for some reason are not safe enough to do the job. It is however, extremely effective in killing all mail order sales of (small quantity of) photographic chemicals.