It's not hidden, it's not subtle: You should be ashamed of yourself.
The 'shameless' isn't even required. Most of the time if you hear someone say the words the distaste is evident in their tone, especially if you hear them spoken in the UK. The US is a little more forgiving.
And sure, there are circumstances where self-promotion can be an unpleasant thing. In an office/company environment, promoting yourself at the expense of others might be a necessary evil to some, or classless and unfair to others.
But for authors? C'mon! We're one person sales-machines. We write books. We need them to sell. We're not stamping on someone else's chance. And who else is going to promote them?
"The readers!" you answer. "If you can't rely on your readers to promote your work ... it must be a bit shit, no?"
Well, yes, word of mouth, one reader to the next, is the best sales mechanism any writer has and moves a lot of books... but why should it end there?
Should we only ever lift boxes with one arm for fear that if we use two we'll be seen to be 'trying too hard'? Or should we use all the resources at our disposal?
I earn my living as a writer. I pay my bills with the portion of each book's sale price that comes my way. Why would I not use the social media options at my disposal to remind people when those books come out, get discounted etc? Who else is going to do it for me?
Self-promotion for authors means book promotion. We're not declaring ourselves to have great teeth and spectacular soccer skills ... or to be better humans than the next person ... we're saying 'buy my book' in as interesting a way as we can.