1 IG + 1 Pinterest account on the same proxy not OK?

I asked a proxy provider if I could use their "Instagram proxies" for a Pinterest account too, but he said that I should use 1 IG proxy per IG account and 1 Pinterest proxy per Pinterest account.
Is this for real? You shouldn't use "IG proxies" to host a Pinterest account?
Sounds like a way to make you buy more proxies, no?