Here is what I did.
1. I've selected an account - ABC Labs on a sample org 
2. Then I went down to "Open Activities" Section
3. Clicked on the "New Task" button which took me to the "New Task" page.
4. Did a search and selected a "Contact" for a Task that belonged to ABC Company
5. I was hoping that the "Account" field will be automatically populated and grayed out once I select a "Contact" since the "Contact" belongs to an account, but the "Account" field is still editable. I can still go ahead and change the "Account" field to an "Account" that's totally unrelated to the "Contact" I have selected. I thought there will be some default validation rules already set up. Am I wrong to make that assuption?