Well, they wanted to leave you with a taste of what 9th level spells were like (hence, being able to cast them from the scroll), but wanted you to want for something more in BGIII or NWN. It's a popular tactic that many companies do. If I remember correctly, it was this way back when they had the gold box series of games too. You can cast the 9th level spells in BGII, just can get a high enough level to cast them from memory. If you install the experience point remover, then you can get to a high enough level to cast the spells. So if it's bothering you a lot, you can download that from
http://www.teambg.com Just go to the player options (not developer) and you should find it there. It looks like there is a link to another page, so you will have to hunt down that page a little to find it. Totally worth it though.