[bartread]

They probably have metrics suggesting that having both sign up and sign in buttons on the page leads to lower sign up rates than having only a sign up button, or something along those lines.

Rather than find a more in depth/better designed solution, it's easier just to remove the "sign in" button and any "confusion" that might cause for users who would otherwise complete the sign up workflow. If their A/B testing indicates that removing the button improves sign up rates, that's exactly what they'll do.

It's super-annoying and short-sighted - not to mention lazy - but this kind of micro/over-optimisation of behaviour on the web has been de rigeur for at least a decade now.

A better approach would be to try to understand why "confusion" around sign up/sign in is happening - i.e., what's the real reason having both buttons/links on a page decreases sign up rate? Root cause the issue and you can fix the real problem in a way that probably doesn't annoy your customers. That's effort though and most customers probably don't care enough to complain about the annoyance of hunting around for a sign in button or link.