The Twitter API users/search documentation clearly states that the search function is similar to the Find People search that's available on Twitter. I did a Find People search with my email address and it searched for everyone that has a @ispprovider.com
(example).
The reason for this is that email addresses are private and confidential user credentials that twitter cannot release under the API (Users log in with their email address if they need to). They can only do search on public, available data that the user shares). If you want to search by email, you can write to Twitter and request to activate the search by email option. That, to me is a dangerous security risk if they do allow search by email.
The Find By Email option is only available in the Find Friends option which basically allows Twitter to go through your email account (GMail, Yahoo, AOL, LinkedIn) , retrieve your email contact lists and bash it to their database to see if there are email address matches.
Twitter never released the "Let others find me by my email address" on Twitter API (as I mentioned earlier, it will pose a dangerous security risk).