There are 4 registered Hereford bulls listed as thf/phaf free in the 2012 Genetic Horizons catalog. Not sure the exact reasoning behind testing those particular bulls. I do know that when the tests first became available, SEK tested their most popular bulls, whether their pedigree indicated it was necesarry or not - I'm guessing maybe that's why those particular bulls were tested.
Th bulls have been siring Hereford marked calves preceding Carpe Diem quite a bit - including his sire Tank, and grandsire Heat Seeker. Even back to (and before) the "double dirty" Stinger. Shorthorn bulls have been known to sire Hereford marked calves as well. As MRW said, it's not violating any rules to use any kind of bull to produce Hereford appearing show steers, for shows that classify by appearance only. Obviously it is to register their heifer (or bull) mates, but it'd be pretty niave to say it hasn't / won't happen...
Of course, bulls like Carpe Diem and Red Rock have greatly increased the odds of a TH carrier getting a calf that would actually pass as Hereford. With bulls that aren't Hereford marked themselves, getting a true Hereford marked one is strictly hit and miss - mostly miss.