Dyrone said:
I think it's the artist's job to let it be known that they are not friendly to reposting their work. If you want to be that weird 1% of artists who posts their work publically on the freaking INTERNET and you don't want others to share it around, then that's your job to voice that considering that's not really normal.

That said when you do repost their art you had better credit them properly and source back to their site otherwise you are a garbage person.

Assuming consent until someone explicitly opposes you is not a very good idea

Joking aside, please do not make massive simplifications based on your own worldview. There are many reasons why artists may post their works publicly and not want them here (SFW artists not wanting to appear in NSFW websites, wanting to retain complete control on which audience has access to it, wanting to retain control if they need to delete it, etc).

If you want a more formal reason, remember that under US law you are absolutely required to get the artist's written permission in order to redistribute their work in any way (also keep in mind that we are adults here, so trying to use "but people don't usually obey copyright law" as an excuse will just make people think less of you and make them more likely to disregard what you're saying)

In the end, a good general rule is "Don't be a dick, if you're not 100% sure just ask the artist"