When the tipping program was launched, anyone could get in if they emailed tapas about it.
Tapas employees then had to sit down and manually activate tipping for every single applicant. As you can imagine, this is an extremely time consuming task. Time that would be better spent towards other things that will build Tapas' future.
The 250 subs unlock point was added so that it would be more practical. It's that simple.
The reason people under 250 subs can't unlock tipping automatically is because it would pave the way for fraudulent activity or people getting tapas accounts just to tip themselves.
Sure, technically people can make that happen anyway... but it would take such an insane amount of effort that the gain would be too small by comparison, and almost no trickster would even bother.
Tapas has opened up to people sending email requests when there were tipping events. If you are below 250 subs still by the next tipping event, you can always try sending an email then.