it is the Standard AX 2012 behaviour of the List Page forms it doesnt the override methods in the form.
Here is the example - created the Form of ListPage, it doesnt allow to override the form methods.
In the case of the normal forms - it allow to override the forms methods
So try by creating the form by having the formtempate - property as None(i.e. normal form) and try to override the form methods and then change theform property FormTemplate to ListPage - to show a message the override methods will be deleted as shown in the below screen
It is the standard behaviour of AX 2012.
Source - http://dynamicsuser.net/forums