Did he go to Walmart or Best Buy, or direct to an MVNO, and get the phone and plan at the same time, as a nicely bundled little transaction? Or did he buy the phone from Amazon (or whomever), with NO sim, then find the MVNO, get the sim, and get it activated? The 2 processes are not at all alike. If he got a bundle, with phone and plan, then all you're really saying is you have alternatives to the big companies and how they handle business. That is quite true. You are, tho, limited to the offerings of the various outlets you research; specifically, both Virgin and Boost, who promote lower end (but QUITE useful) phones and less expensive plans, don't have a very wide range of phones to choose from. When you say, pick a phone, I read it to mean free choice of every model of phone marketed in your area...so Moto E, Alcatel Idol 3, Samsung A3 or A5. Then pick the MVNO. Still might sound simple...but if you are trying to use Amazon or eBay, you'll soon learn it's not. There are numerous SKUs...you can get US market or international market. They support different LTE bands. The Moto E has 3G and LTE variants, with different processors. Same with the Moto G...there's 2 or 3 variants that aren't clearly different until you read down deep into the specs.
You just said "go buy your own phone." It looks like what you meant was, go get an unlocked phone...and you can do this at many places that will offer unlocked phones compatible with their network, with a sim that'll work, but isn't carrier-locked.