GSIHE - NL Overview Question

[BigBluffer]

I just received GSIHE and immediately dove into the NL section after reading so many good things about it, especially the short-stack strategy. I have never played cash game NLHE, so my mind is completely open in this area.

In the overview, Ed presents some betting/raising constraints. #5 tells us that if someone raises all-in, but that raise is less than the minimum raise for that game, his action is considered a call. Ed gives an example where I open the betting for $50. One player calls and the next player raises all-in for $80. If everyone folds to me, Ed tells me I CANNOT RERAISE. This seems counter-intuitive to the concept in NLHE that "you can bet any amount at any time." The only reasoning I can think of is that since the raise was for less than the minimum raise (which must be equal to or greater than the initial bet of $50), it is considered a CALL. Thus, when the action gets back to me, I cannot re-raise my own raise and thus can only call the additional $30.

Typing it out, I think I just came up with the explanation of why point #5 is correct. Could someone with some NLHE experience jump in here and confirm or correct my explanation?

Thanks.