Copyright has to be registered in order to collect royalties - otherwise the collection agencies - who monitor radio plays, sync usage (music played on television or film), internet streams, etc. - don't know who to pay. So if the songwriter's name hasn't been registered against the song, then they won't get any money.
In the U.S. a work is legally copyrighted when it is written. If the writer can show that he wrote it, then he owns it. The legal action over it might be interesting.
^ The songwriter has their statutory right automatically, but in order to get paid for it you pretty much have to register.
It is the responsibility of the songwriter to register their claim, not the performer - the complication comes where there are multiple songwriters involved (particularly when producers get credits, sometimes having only tweaked one or two minor things). So there is room for confusion here.
There are different collection agencies for different areas - PRS and MCPS are the main ones for the UK (MCPS handles sync mostly, and PRS everything else). I don't know who services the US market, but if you find out who they are their registration processes and guidelines should be freely available so you can find an appropriate loophole.
ETA: I hope "millions" is an exaggeration! Even huge number one hits don't earn that much for the average songwriter (esp. given how little you have to sell for a number one these days). The ones that really earn millions are the ones that are popular for years and years and years - the ones that people choose for weddings and funerals etc.
It's worth remembering that things like Youtube and Spotify pay an insultingly low royalty per stream - it's something like £0.00076p per play, and doesn't get paid until after a few hundred thousand plays or something. Bandcamp is a little better - a friend of mine had 176 plays last year and earned 47p, which works out at 0.0027p per play. MEGABUCKS!