The code you are using isn't actually looking at persisted data. You're essentially creating a "new"  local WTPartUsageLink and I'm guessing 1.0 is the default value. If you would like to actually retreive the quantity from an existing link in the database, you need to get a reference to that link.

One of the APIs you could use would be:

WTPartHelper.service.getUsesWTParts(parents, configSpec);

JavaDoc:
getUsesWTParts

Persistable[][][] getUsesWTParts(WTList parts, ConfigSpec configSpec) throws WTException

Navigates from many used-by parts to their uses
part masters and applies a ConfigSpec to select the iterations
of the uses parts. Returns a three dimensional array of Persistable
where the first dimension is corresponds the the used-by parts passed
in. The second dimension corresponds to the part usage links from
the used-by part. The third dimension is two elements long where [0]
is the WTPartUsageLink and [2] is the uses WTPart or WTPartMaster
if the config spec did not select an iteration of the uses part. This
API does not support filtering. 

Supported API: true

Parameters:

parts - Ordered list of used-by parts.

configSpec - The config spec to use to select the uses part iterations.

Returns:

Persistable[][][]

Throws:

WTException

Once you have the link you can use your code above to get the quanity:

puLink.getQuantity().getAmount();

You could also check out this article to show you how to get the link:

https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS49806