Not always legally. I've begun seeing signs posted forbidding the act.
Generally (not legally) speaking... if you use toilet chemicals, you may wreak havoc on the septic system in place. So I would only dump into approved facilities.
We do not use chemicals, so the toilet contains the exact same thing that we would physically dump when we use the toilet. I can not see any reason for not doing that. We have met people who didn't want us to dump in a normal toilet because they feared we would leave it dirty, but that is a different story.
Instead of chemicals, we use the SOG system, which is very popular here:
http://www.soguk.co.uk/Added benefit: no pressure buildup in the cassette when changing elevation.