It's just as easy on Firefox OS. Writing a web service is not required! Mozilla provides one for you :)
BTW, both iOS and Android require servers to be available for purchase checks unless, as the app developer, you don't care about piracy (which might be legitimate for the "most people will buy it" argument).
As for JavaScript being easy to patch. Is it really easier though? You can literally copy around apk files or search for them on pirate sites to get paid Android apps for free. There are videos on youtube for how to Patch paid iOS apps with fake certs and DNS hacks.