1) Pretty much each and every action has a fee attached [1].
2) Each rejection has a 3 - 6 month time limit to be responded to.
3) Most applicants will fight every rejection to preserve their rights.
4) Granted patents have (admittedly higher [3]) fees due only every 3, 7 and 11 years, whereas patents in prosecution typically have fees due every 3 - 6 months. However, granted patents are not guaranteed to be renewed, which means their contribution is reduced [4, 5].
So if you want to talk perverse incentives, the USPTO actually have greater incentive to issue more rejections and collect frequent short-term fees on applicant responses, than they do to issue patents and collect infrequent long-term fees on renewals.
[1] http://www.uspto.gov/web/offices/ac/qs/ope/fee031913.htm
[2] http://www.uspto.gov/web/offices/ac/qs/ope/fee031913.htm#mai...
[3] http://www.patentlyo.com/patent/2010/06/patent-maintenance-f...
[4] http://btlj.org/data/articles/20_04_02.pdf
[5] http://www.uspto.gov/about/stratplan/ar/2011/mda_06_01_03.ht...