Example: I'm trying to add <input type="checkbox" name="checkbox" checked="checked" /> to the HTML in my Pushpage, however when I click "Update" this does not update/save.
HTML input tags won't work in Pushpage email text editors. There are a variety of text editors used throughout the site, some of which accept input different from others. For example, report card opening/closing text fields don't accept input tags or tables.
With Pushpage, <input> HTML isn't accepted because those are form properties. Many email clients will respond poorly to finding form properties in an email, oftentimes identifying the email as a scam email and sometimes affecting the display negatively. This would display messages to the users receiving these emails that would likely scare them away from opening it and/or cause them to mark it as spam, which would blacklist their email address, so preventing these from being added to a Pushpage helps keep that from happening.