I'm trying to investigate this issue.
I think show captcha on API requests cloudflare not poloniex. Maybe poloniex admins made tougher rules on cludflare account. I don't know reason but I don't found IP's in my city without captcha.
Solution for this use proxy, VPN etc. Many free proxies return normal polo API response without captcha or use hosting for your app in different location.
Create ticket on polo support site few days ago, no answer.
yes, a long time ago it happened with one user and they told me about it being with cloudflare. But it's really bad.
And an user reported me that using vpn my works well, so it's a valid solution.