Is it unethical for professional societies to offer conference discounts to members? There is a recurring pattern I see in conferences organized by several professional associations. Two different conference fees are offered, X for members and Y for non-members, with Y>X. Often, the annual membership fee for said professional associations is less than Y-X, so becoming member is cheaper than paying the non-member fee. For example, for different reasons today I have stumbled upon the pages of this, this and this conference.

I can only see two possible reasons for this practice, both ethically dubious:


to force people to become members, increasing artificially the dimension of the professional association.
to "move" funds from the conference treasure to the association treasure, leaving them available for a larger number of activities.


Often conference fees are paid by research funds, while membership fees are paid personally by the researchers, so this practice also has unpleasant side-effects on their personal finances.

Questions:


Am I overlooking more plausible justifications for this practice? Do you agree with my analysis?
How ethical do you find this practice?
Should I raise the issue with the professional societies I am a member* of?




*: You can probably guess the reason why I am a member. :(
Typically the organizations that do this are professional societies. And, as you mentioned, they have many more activities than just running the conference, so having membership fees separate from conference attendance fees allow them greater flexibility to manage their accounts. 

Some places allow membership fees to be paid out of institute funds (this depends on the country and the situation, of course). However, many countries also make such professional expenses tax-deductible (after a certain threshold). Obviously it doesn't recoup the full cost of the membership, but it's better than nothing.

However, one additional point to consider is that most such conferences require that at least one of the authors of a presentation to be a member of the society. (Sometimes, this requirement falls on the presenter herself.)