Normally only Leaders (Command Teams) would be able to rally troops. So why tanks too?
This has again to do with abstraction and your imagination filling up the gaps left by the ruleset. Imagine tanks not actively ralying accompanying units but rather boosting their morale. The appearance of a friendly tank nearby may be sufficient to get up fighting again.
This is also the reason why tanks will automtically have to attempt to rally as many troops as possible (up to three if they're in command range). It is also the reason why tanks, other than leaders, are capable of rallying while in close combat.
This rule also underlines a tank's special role in the game and it may become a supporting pillar of your tactics - at least more than the average infantry team.