The only real difference between an MD and a PA is that an MD working as a General Practitioner is wildly overqualified for the job whereas a PA working as a GP was trained specifically to be a GP. It's like the difference between an Opthalmologist and an Optometrist. An Opthalmologist is perfectly capable of getting you a new pair of glasses but that would be a waste of a lot of his schooling if eyeglass prescriptions was all he did. PAs are allowed to write prescriptions, even for controlled substances. The MD's name should also appear on the prescription because a PA is acting as the MD's agent, but a PA doesn't have to go get permission from the MD to write a prescription.
As for the Lexapro, it's also an anti-anxiety medication. Regardless, there are only two types of prescriptions, general prescriptions (like Lexapro) which they just fax over to your pharmacy, or controlled substances which must be recorded in a DEA log with the prescribing physician's DEA number. You also can't call in or fax a controlled substance prescription, you have to physically pick up and take the original copy of the prescription to the pharmacy every month. There's no specific taboo on GPs writing prescriptions for 'mild' stuff like Lexapro or Zoloft, although they probably wouldn't write a prescription for a heavy duty anti-psychotic like Geodon or Seroquel unless you'd been a patient for a long time, and even then they probably would have already referred you to a psychiatrist.
Incidentally, they're called Physician Assistants, not Physician's Assistants. A PA is a physician who is also an assistant, they are not only an assistant to a physician.