How can I remove decimals when I award user points via commerce-order-total:amount
Have a look at the Drupal Commerce issue about "Revise the way we handle price amount values". Especially comment nr 37 in it, which states:


  The price field is and will continue to store price amounts as integers ...


So rounding an amount of 1500 dollarcents is ... still 1500, right? And that is the amount of user points you're granting. And because of that, your rule works as implemented: you're using that value of 1500 as the amount of userpoints to be granted.

Obviously, all you need to do next is to expand your rule to devide the price by 100 first (before rounding it). I assume you know how to do that, if not use this link ... bonus hint: have a look at the example rule included in the answer to "How to add up all items of a field collection field using Rules?", especially the "Calculate a value" Rules Action.