Lines 111 to 113 of qtbase/src/network/kernel/qnetworkproxy_generic.cpp set http_proxy as the default source for a protocol proxy in case no specific protocl proxy was found.

I have turned on -system-proxies in Debian both in Qt4 and Qt5, and started receiving bug reports of users wanting to use a proxy only for http but for other protocls like, for example, IMAP. But they found out that setting http_proxy would make their apps try to use the proxy for everything.

So I guess the "culprit" here are the lines mentioned above.