[unclebucknasty]

There's always the possibility that you won't necessarily always want to direct so much of your shopping on Amazon as to recoup that Prime cost. A new player may enter or a current one change such that price, selection, service, etc. makes it more compelling to direct more of your commerce through them. There is also the possibility that Amazon itself changes to be less attractive as a commerce option.

This move by Amazon is absolutely about lock-in. Get their tentacles into as much unrelated to commerce as possible and you will likely keep your commerce with them too.