Because even with no promos, you earn 600 Southwest poitns per rental, which are worth $10 until next spring (and still over $8 after that).

$10-$4 = $6 profit.

But not all frequent travel programs incur that kind of charge. I don't tend to get it, for example, with hotel points, including the common promo with Club Carlson for 7500+ points for a 3-day rental.

And not all airlines incur the same amount of charge.

And it's more worth it, obviously, if you have a promo which will earn 2x or 3x or 4x the normal amount of Southwest points (or whatever). In that case, it's still the same $4 (or whatever) surcharge, but the surcharge gets less signficant the more you earn in miles/points on the rental.

But yes, that does bring up the point that it pays more to "follow the promos" (ie, always use whichever program you participate in that has the best promo), rather than always using the same program (which is not likely to always have a promo).