The issue with P2P
Broadly speaking, network components share network resources amongst users on a per TCP connection basis. UDP transfers are typically expected to be brief (e.g. DNS queries) and therefore, insignificant. This works fine with traditional applications like http and ftp, where a single TCP connection or a few TCP connections are used per application. However, P2P applications effectively "cheat the system" by establishing large numbers of distinct TCP connections with multiple destinations. Many P2P applications also use UDP, again using multiple "connections" with multiple destinations. The end effect is that P2P applications grab way more than their fair share of available network resources, resulting in poor performance of other non-P2P applications and complaints from the affected users.
VPNs or applications using nntp, ssl or ssh on single TCP connections are limited by the normal mechanisms, won't grab unreasonable chunks of available resources and are therefore, unlikely to attract attention for reasons of bandwidth usage.