Nobody is testing for fun. People get tested if they have symptoms. The positivity rate is a key metric as is estimates how much of the population is getting covid vs. a common cold. It's a huge reach to say that Lakewood and BP are being "smarter" than everyone and only testing people who were symptomatic.
To your other point, both metrics (number of new cases and positivity rate) tell part of the story. Taken together, they offer a good window into the state of things. If the number of cases is increasing, AND the rate of positives are BOTH increasing, that's a clear sign of an outbreak.