Concierges were the original travel agents (in Britain they were called head doormen). And they work on the same commission basis from places they book you into, or an equivalent if you pay your own bill, which is why they always go back to the same suppliers every time. So do not worry that they are not already being paid for their services.

While tipping for assistance over the norm is reasonable, the habit of tipping (or overtipping) them universally is something that has come across the Atlantic in recent times. Fortunately the habit of following this up with ramming the amount you tipped down everybody's throats when you return from your travels with your stories has not made it into European society. Yet!