only when publicly accessible yes. but twitter recently disabled reading tweets without an account and bypassing that for web scraping could make it not legal
Depends very much on the terms in which the content is made available. It's more than a little over-zealous, but the Computer Fraud and Abuse Act technically makes any unauthorized access to a computer system illegal. If Twitter makes it explicitly known to you that you're not authorized to scrape them and you do it anyway, there is some potential liability.