(Most) Japanese boards use '◆' for the display character. 4chan started using '!' instead because the diamond looks really clunky and retarded when it's not next to a bunch of kanji/kana characters. Oh, and WTFux uses a '~', but they're a bunch of furfags anyway, so fuck them.
As for why a different character to enter them than the one used for display, consider this: the text that's displayed on the page is also different than what you typed. Or would you rather make those the same as well?
Incidentally Wakaba/Kareha do allow using a '!' to delimit the tripcode key.