pseudoephedrine log

i was pulled over for a traffic violation and the officer informed me that i had a warrant for dangerous drug violation pseudoephedrine purchase violation. what is the legality of law enforcement viewing log books of different pharmacies "fishing" for violations. i wasn't aware of the 1 purchase in a 24 hour period i thought it was so many milligrams in 24 hours i purchased regular release at one pharmacy as they didn't have my usual 24 hour tablets. I then found another pharmacy and purchased a box of 24hour tabs not going over my milligram limit for a 24 hour period. i was arrested and had to post a 300 bond. any comments are appreciated