I don't think you want your boss to replace you when you go on vacation.
If you are one of 100 CSRs, then it wouldn't be business-destroying obviously.
Business owners can take this risk but using "don't take more than 2 weeks of PTO" as a risk mitigation strategy is disingenuous and disrespectful to their employees.
Haven't you ever joked about the "bus factor"? All the startups I've worked at would have likely failed if any one of the core people had to depart, for any reason.