According to their post on Google+, they initially wanted Carbon on Android to be a paid app. However, UAE is not on Google list for paid apps. So, they waited for Google I/O recently for new announcement and again, there is no UAE in the list.
Therefore, in order not to let everyone wait, they made a decision to provide this premium app free for Android users. They target to submit the official app on the week of 22nd July as version 1 (instead of a Beta version).
or watch a teaser of Carbon on Android.