What? Attacking Israel is okay as long as you don't attack anyone else--then they're not terrorists? Have you taken leave of your senses? And did you miss Hamas' war's w/ Lebanon? Get a grip man!
These parties have been at war
since WWII. The Hamas-Syria-Israel conflict is a simple border and territorial dispute similar to those that occurs regularly in Europe or the struggles of the American natives against the white invasions, the American militias etc. Different weapons, different continents and era but the goals were similar in each case. Yeah, violence and guerilla tactics were used , casualties mount but when negotiations break down and tempers fly, that's what you get.
On the other hand, a terror group has no defined goals other than the destruction of the enemy. A terror group does not abide by UN mandated ceasefires.
Oh, BTW I never said attacking Israel is "OK".
Besides, your premise is wrong. Not every group who attacks another is automatically lumped
into the "terror group" category e.g. Serbs, Bosnia attacking the Soviets, Afghans likewise, Tibetans "attacking" the Chinese and vice versa, Native Americans I mentioned above etc.
The reason is that they wrongly or rightly (decided by which side you are on) are deemed to have a "just" cause.