Get into on your icon file.

Select the icon, copy it, paste it over onto the webloc file as you're doing.

Basically, don't paste the file onto the icon, copy the icon of the icon.

Follow these steps:

1. Get into on your icon file (open it for example with Preview).

2. Select the icon with Command + A.

3. Copy the selected icon with Command + C.

4. Paste it over onto the webloc file (as you're doing) with Command + V.

Basically, don't paste the file onto the icon, copy the icon of the icon.