If what they say is "if you add this extra functionality without our permission, we will sue you", it is not OK. This cripples peoples ability to innovate.
If they on the other hand say "This is the basic functionality, and you can buy this add on package for $400 if you want extra functionality", I have less problems with it. The latter opens up for third party addons, possibly being more innovative than the original addon software. What I find abusive is the attempts to prevent people form innovate using the law.
If what they say is "if you add this extra functionality without our permission, we will sue you", it is not OK. This cripples peoples ability to innovate.
If they on the other hand say "This is the basic functionality, and you can buy this add on package for $400 if you want extra functionality", I have less problems with it. The latter opens up for third party addons, possibly being more innovative than the original addon software. What I find abusive is the attempts to prevent people form innovate using the law.