Why should a company be allowed to disallow
well, why would they be disallowed to disallow?
I guess the primary reason for disallowing is to protect their bottom line (cost reduction), sold as "SECURITY!!!!". In short, if too many people play around with exploits, apple would actually need to deal with this (= cost). And please don't tell me they could make it clear in terms and conditions, people don't read anything, particularly T&T. Very likely, some idiot(s) try to install something "alternative" off the internets, that turns out to be malware, and then they sue apple for billions "because they did not provide basic security features".
And certainly, secondary reason is that the walled garden / ecosystem adds that extra profit.
p.s. while I hate apple as much as anybody, exactly the same goes in the android universe - and fewer and fewer phones are rootable these days.