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My understanding - based on my experience with PenFed and Apple FCU - is that credit unions generally require HP's for customer-initiated CLI's, probably because CLI's are often classified by them as "seeking new credit". In fact, there aren't any "luv buttons" on either of those CU's sites, you have to fill out a complete new credit application for a CLI and specify that you're asking for one and how much extra you want.
SPs are used for more than just auto-CLIs. This week I received an auto-CLI on my AMEX BCP card, plus approval for a new AMEX Platinum card, all based on only a SP.
AMEX presumably considers this simply investing in and deepending an existing credit relationship, so no HP required.
Few other card issuers would do this without resorting to a HP. Each lender seems to have its own rules.