I have seen that there are extensions brand ©
Is allowed to remove that line of code?

In the part of policy is:
Joomla Extensions by joombig is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

But I have tried some extensions are in GPL2, in which case it would be allowed.
Also in joomla directory they say: license is GPL.

I have to understand that you do not give support to remove that line, or not permitted to delete it?

Hello,

thanks for visit our site,

Please, download Pro version that will remove copyright and support all problem

thanks & regards

This means that i can remove on my own without law problems?

Hello,

Yes, If you using Pro version, that we will support you all problem

thanks!