Do all Rental Deposit Agencies simply multiply by 1/2?

I've only had a few interactions with them (thank the lord) but they all seem to point to the same conclusion...

Rather than do the ****ing job that they're supposed to do, they just seem to take whatever number is requested by the landlord and divide by two. No ****ing investigation, no telling the landlord that £500 to clean a terraced house is excessive, no looking at inventories, no looking at photos/videos on the property on move in date....

Just give the landlord a huge chunk of money 

Tell me it ain't so.....