I think “Note 2” on this knockout page explains it all:
http://knockoutjs.com/documentation/click-binding.html
You can avoid the problem by either using:
Anonymous functions:
<button data-bind="click: function(data, event) { myFunction(data, event, 'param1', 'param2') }">Click me</button>
… or …
The bind method:
<button data-bind="click: myFunction.bind($data, 'param1', 'param2')">Click me</button>