That's exactly how it works. Your two isolated incidents has caused you to develop a theory with no investigation to confirm your theory. You've then told people not to take an action because of your untested theory. Let me counter both of those individually.
First, the eCig. How did you determine the cause of the reboots was the eCig was drawing too much power? How much power was it drawing when it shutdown? How much power does it draw when hooked into a wall adapter? Does it do the same thing plugged into a different computer? Was it plugged into the front or rear ports? I can't say I've ever experienced a USB device shutting down a computer, but from everything I've read Windows, Linux, and OSX will tell you a device is drawing more power than it's supposed to and shutdown the port or hub. More importantly, Windows and OSX (not sure about Linux) will also show you both how much power is available to the ports and how much power the device is asking for. But it might not have been a power problem either, at least not on the eCig side. If the device has a chip to negotiate, it's entirely possible it was just a crappy/incorrectly configured chip. Or, also consider the fact there's been reports of the Chinese e-Cig's being encoded with Malware. IF, and I stress the word IF, the eCig drawing too much power caused the PC to shutdown, that's as much the fault of the PC as the eCig. They are supposed to use polyswitches to cut power when draw exceeds maxes.
Second, the HP Stream. That's a shortcoming with the device. They are known for drawing too much power even when turned off. That's why battery life is so short even when powered off. It's not that it's not being provided power (aka "charged") by the computer, the tablet is just using power faster than it's getting it. Two very different things. If I turn on the GPS on my Galaxy Tab and drive out to the desert, I can get it's power draw high enough that it will lose charge even when plugged into Samsung's car charger (Cig lighter adapter or USB port). But that poses zero danger to the PC (or my car). Both devices are functioning exactly as intended. That said, a quick search shows multiple people stating they are able to charge their Stream 7 off a PC's USB although at least some reported having to lower the screen brightness to get a noticeable charge off of it.
Last but not least, a quick search shows plenty of people charging both eCigs and Stream 7's off PC USB ports. I see multiple eCig manufacturers saying you can change them off your PC's USB port. More than enough to make your blanket statement completely false. Finding ONE USB device that caused a problem is not enough to tell people nobody should plug that type of device into their computer. Especially when you don't actually know what causing the two devices to interact improperly. I've got a Startech UPS console cable and whenever I plug it in to my APC UPS, the UPS shuts off. That doesn't mean I go around telling everybody not to plug console cables into their UPS.
Not to mention there's literally millions of higher powered USB devices that are charged off plain old USB ports all the time. I've got 3 different tablets that I've charged off my PC's USB ports. My phone is plugged into my laptop's USB port for power all night at work while I watch Netflix on it. Right this second, my NZXT X61 is requesting 50mA, my HTC M8 is requesting 100mA, my G700s is requesting 498mA, and my G510s is requesting 500mA. I've also got an Asus Blu-Ray burner that uses a Y-cable to draw power from two ports to ensure it's getting enough. I can't find a single verified case of a computer being damaged from a USB device drawing too much power.
