The event is being fired from core. You can disable this event observer in a custom module (rather then editing core files, which is evil ;) ). Put this in your `app/code/local/(VendorName)/(ModuleName)/etc/config.xml`:

    <config>
        <frontend>
            <events>
                <catalog_controller_product_view>
                    <observers>
                        <reports>
                            <type>disabled</type>
                        </reports>
                    </observers>
                </catalog_controller_product_view>
            </events>
        </frontend>
    </config>

The magic happens in merging your `config.xml` with the one from core that is defining this observer. Your XML overrides the core `<type>` node and sets the type to "disabled" which results in the event observer still being there, but not being fired.

> Is there any downside to this (*disabling viewed products counting*)?

I can't think of any downside in disabling this other than that you can't see product view reports anymore...