Why do you typically have to pay for use of the washing machines while other amenities are free?
As you said, you would prefer to have free washing machines instead of free swimming pool. What does this mean? That washing your clothes is more important than having a bath in a swimming pool. Usually, it applies to almost anyone. So you could stop using swimming pool, but not washing machines.

Also, it does not seem probable that many people will use that swimming pool at the same time. But even if they do, people will not complain about massification unless it is too much and permanent (both conditions are rare to be filled).

So:

1) They know that washing clothes is more important than having a bath in a swimming pool. And even more, they know that you cannot avoid to wash your clothes (but you can avoid having a bath).
2) They do not expect the swimming pool to be always full of people, so they do not expect many complaining about it.
3) Having a swimming pool is cool. Having a free swimming pool is very cool. Marketing.

In conclusion: they use the swimming pool as part of their marketing, while they make money from washing machines since you have to wash your clothes with no potential alternative (while having a bath in a swimming pool has several alternatives, so if you had to pay for it, maybe you would not use it).

I hope it helps.

EDIT:

For those who do not know what I am talking about, take a look of these two terms: "utility" and "elasticity" ;)