Pirate or unapproved App Store replacement apps must jailbreak the Apple device to function correctly, because they use privilege escalation and circumvent Apple security to install apps or grant many security permissions. Some alternative apps claim to not require jailbreaking, but they do not have feature sets comparable to Cydia.
Jailbreaking Apple phones may sound illegal, but it is actually legal in Canada and the United States as of 2015, although the legality is disputed elsewhere in the world. It is the piracy of apps through unapproved App Store replacements that is most often illegal by by violating copyright laws. It is especially important to stick to legal channels for finding apps, as some unapproved App Stores are known to distribute malware.