$10K players cost money, academies cost money, when you start adding it up, the Phillies have invested significant sums in LA.
I don't think they had spending constraints this year so much as the top players were already tied up, so there was no reason to overspend on second tier guys and get squeezed under the new scheme. Given the numbers this year, it looks like they spent just to the limit but spread it around a lot of players, then continued their usual "$10K" strategy to look for diamonds in the rough - they've found some legitimate talent this way, not just LA roster filler.
A lot of LA roster fill signings are 19-20 years old, mature enough to help a LA team win, but too old to have much future (except for the occassional sleeper pitcher).
17 year old signings are more likely "long shot" signings, players who were overlooked at 16 years old but who might have physically matured and showed some flashes.