The first time, years ago, that I saw "Pietasters", I misparsed it as people who are bad at pie, or piety?, on the analogy of poetasters (poet-asters, see).
"-aster: a. L. -aster, suffix of sbs. and adjs., expressing incomplete resemblance, hence generally pejorative (Diez); e.g. L. philosophaster a petty philosopher, oleaster a wild or bastard olive, surdaster a little deaf. Extensively used in Rom. langs. (It. -astro, Sp. -astro, -astre, Pr. -astre, OF. -astre, mod.F. -âtre), esp. in F. as adj. suffix, e.g. bleuâtre bluish, blanchâtre whitish, etc. In Eng. only in words from L. or Romance, e.g. astrologaster, grammaticaster, oleaster, poetaster, politicaster."