This is no surprise and many of them will be doing this, as the time and effort to find out the airport and govt charge at each point is way too much time and trouble. Given that FR fares are still so low, few will argue.
However, it is not just FR ... I was weighing up using Air Miles™ on a simple LON~AMS. As you may know, the UK company Air Miles™ are agents for (mostly) BA, so I also checked the price of buying the same flight direct from BA. I found that the various taxes and charges did not align.
To get the breakdown from AM, I had to make a special email request as the web site just lists the total without explanation. A week later I got the info and the costs varied in almost all areas and were higher for the AM ticket, which would of course have been the exact same rotations. This confirmed my suspicions but, by that time, I had already booked on EZY. Not to say that they would have been any better.
Rat 5
I thought that EU law was coming in whereby advertisers had to publish the final price, incl VAT and all charges. What has happened to that?
As I understand it - they all print the costs and a paragraph of explanation but DO NOT give a detailed breakdown of the components of that total. Hence they meet the current regulation of ensuring that the purchaser knows the full cost. As always, the free market is ahead of regulations.