Mob spawning and block place prevention turned off by default?

[Onyxtite]

Hello!
I was recently configuring the plugin and noticed that this two options were disabled by default:
"prevent-block-place: false
prevent-mob-spawn: false"

Do they negatively affect performance or generate any issues?
I'd just assume that most people would use them otherwise, so it got me a little worried about activating them.

Could you confirm this isn't the case?

Thanks for your time!