Why does the PNG specification prohibit having zlib-compressed data with a preset dictionary?

Question

For PNG compression method 0, the zlib compression method/flags code shall specify method code 8 (deflate compression) and an LZ77 window size of not more than 32768 bytes. The zlib compression method number is not the same as the PNG compression method number in the IHDR chunk (see 11.2.2 IHDR Image header). The additional flags shall not specify a preset dictionary.

The PNG specification prohibits having zlib-compressed data with a preset dictionary in PNG data. Why does the PNG specification require that?

Answer 1

Because a PNG file should be decodable with no other information.