Why ids in urls don't work but slugs do?

Question

I'm lost, I don't understand why this works

http://website.com/?tag=my-cat-slug

and not this

http://website.com/?tag=15

It's really annoying because wp_dropdown_categories() only provides ids as values.

I tried to change permalinks to default settings instead of rewriting:

http://website.com/?p=123

But it changed nothing