http://www.google.co.uk/search?q=site:.be+advertising+agency shows that they're some of the top advertising agencies operating in Belgium.
They're complaining not about not getting clients but about getting potential clients who insist the agency outlay a large amount of time and resource before committing. This is simply abusive as all the ad agencies work is out in the open for the client to determine if they like it and so the pitch is simply about deciding if the advertiser can work with the agency.
It seems a reasonable complaint. Trying to return a sense of decency to business practices is probably a tough road though.