[markasoftware]

Residential proxies operated by "legitimate" providers can only open TCP connections, which more or less rules out DDoS attacks, which seem to be the most common use of botnets. (And all tcp connections typically have to go through the proxy providers datacenter first to get through NAT, which effectively limits the total amount of traffic/connections you can make)

[account42]

TCP only does NOT rule out DDoS, it just limits it to a subset of potentially less effective methods. All DDoS means is that you get a bunch of machines hammering a system.