Stealth errata fixed that
The premium edition Player's Handbook (2012) added a "max +20" clause to both Transformation and Divine Power, effectively turning your premise false.
However, how good is a rule that only those who bought the book (or who got told by someone on the 'net) know about?
Basically, while this errata solves the problem even better than my previous version of the answer and while it looks very intentional*, not everyone even knows that the rules have changed (not even the d20srd site, which is usually good at errata-ing things).
*In this case they added words, in some others pages it appears they forgot that there was an errata out already and they ended up unmaking some changes for apparently no reason.
Anyway, even before the stealth errata Transformation (and its more useful divine version that does not prevent spellcasting, Divine Power) follow the general rule that mandates that you don't ever get more than 4 attacks from you high BAB: it is not "one attack every 5 BAB points, round up", it is "one more attack if you have 6+, one more at 11+, one more at 16+"